Sath = that which is true.
Guru = remover of darkness (ignorance); a teacher.
So Sathguru = he who removes the darkness of ignorance by bestowing the light of true knowledge.
To put it simply, a Sathguru is a true teacher because he gives us knowledge of God, the universal Truth.
Life is full of learning. We learn by being taught. We learn from our parents, siblings, friends, teachers, books, educational programs on TV and so on. Thus even our material knowledge is gained from other sources. We don't generate it ourselves. So we go through life by learning from many teachers. He who teaches you mathematics is your mathematics guru and he who teaches you economics is your economics guru. But these gurus are not Sathgurus. The Sathguru is one who can give you the experience of God.
A Sathguru is qualified to teach us about the universal Truth because he has personally experienced it. Talking about God is one thing, but experiencing him is an entirely different thing. The spiritual world is a world of personal experience, not of talk. The Sathguru, having experienced God is qualified to not just talk about God, but bestow that ultimate experience on others.
God won't reveal himself directly to his devotees. He will reveal himself to his devotees only through a Sathguru.
A Sathguru is a Siddha who has been commanded by his Sathguru to convey the knowledge of God to others and to work for the spiritual upliftment of the world.
A Sathguru hails in a lineage of Sathgurus. This lineage is called guru parambarai. The lineage ensures proper transmission of divine knowledge from Sathguru to disciple. The Sathguru picks a disciple who has attained spiritual perfection and commands him to continue the lineage by appointing him as the next Sathguru.
Of all the powers of God, the power that's most most valuable to humankind is his power of grace, arul sakthi. The Sathguru is a personification of God's grace. He is an emanation of God. The great saint Manivaasaga says that he worshipped God not of his own accord, but because of God's grace. Without God's grace, one cannot even think of God, let alone reach him. The devotee must understand that everything achieved on the spiritual path is a result of God's grace. Since the Sathguru is the human manifestation of God's grace, our spiritual progress depends on the Sathguru's grace. How did the Siddhas attained Siddhahood? Only by the grace of their respective Sathgurus.
Thirumoolar emphatically states that the Sathguru is God. If you want to become a Siddha, treat your Sathguru as you would treat God.
The Sathguru-disciple relationship is the time honored way of learning about spirituality.
The Sathguru is God's gift to you. It is God's way of making himself immediately accessible to the human mind. The human mind is unable to comprehend God because it can only comprehend that which is perceptible to the five senses. God is beyond the five senses and hence beyond the grasp of the human mind. To solve this problem, God takes human form as the Sathguru, a form which can be comprehended by the human mind. This is God's gift to us. He comes down to our level just to lift us up to his level. Spiritual aspirants of the world, remember this act of grace. Meditate on it, make use of it and reach the Merciful One.
The Sathguru, being a part and parcel of God, is a representative of God. Imagine how you would deport yourself in front of God if he were to appear before you. Deport yourself in exactly the same way in front of your Sathguru.
If divine love is the way to reach God, why does one need a Sathguru? Because it's the Sathguru who makes divine love blossom in you. The Sathguru's grace makes that love manifest itself as service. The Sathguru blesses you with the name of God which makes you excel in service. It is true that the holy trio of love, service and the divine name can take you to God, but it's the Sathguru who blesses you with this trio. He transforms you into the fertile bed in which this trio can bloom and yield the ultimate fruit of spiritual perfection. This is why we need a Sathguru. If you are a ship travelling towards God, the Sathguru is your rudder. Just like a ship can't make progress without a rudder, you cannot make spiritual progress without a Sathguru. This is why Thirumoolar says that without a Sathguru, one cannot reach God.
The Sathguru gives you the seed, not the fruit. You have to plant the seed, water it and care for it in such a way that it becomes a tree that yields fruit. In other words, you have to do the hard work that transforms the seed that is God into the fruit of spiritual perfection. The Sathguru will give you advice during this long, arduous process, but it is you who has to put in the hard work if you want to taste spiritual perfection.
If God is the goal of life, why is it that so few of us are interested in him? Because we have succumbed to the temptations of the material world. To look beyond these temptations and focus on God, one needs clear thinking. The Sathguru blesses the disciple with this clarity of thought. Thirumoolar explains this by saying that one gets this clarity by looking at the Sathguru, by listening to and acting on his words, by meditating on his name and by thinking of him. Without this clear thinking, a disciple reverts to the temptations of the material world thereby jeopardizing his spiritual progress. Hence clear thinking is an absolute neccesity on the spiritual path and only God, in the form of a Sathguru, can give this to us.
Respect the Sathguru, revere him and worship him. Heed the Sathguru's advice, have faith in him and obey him. The Sathguru, having attained spiritual perfection, is immersed in God consciousness and is therefore unmoved by both praise and scorn. But the relationship between the Sathguru and his disciple is so holy that God has laid down some rules regarding it. The rule that is binding on the Sathguru is that he should protect and care for the disciple. The rule that is binding on the disciple is that he should respect, revere and worship the Sathguru. God rewards the devotee's attitude of supplication towards the Sathguru by showering him with divine grace, the key that unlocks the treasure chests of the spiritual world.
Since the Sathguru is the rudder of the ship of spiritual life, every spiritual aspirant needs a Sathguru. But in the Kali Yuga, the present age, it is not easy to find a Sathguru. How then can you find your Sathguru? There's one way. Surrender yourself completely to God and request him to show you the way. Do this constantly with sincerity and God will personally lead you to your Sathguru.