Hey Bhaarata (Arjuna). It is Faith which teaches renunciation and is responsible for the elevation and well-being of human beings. Whenever in the passage of time,(yadaa yadaa) Faith (or dharma) is weakened or is under attack, (glaanirbhavati) - and whenever adharma spreads without control (abhyutthaanam.h) - it is then (tadaatmaanM) that I re-incarnate myself (sRRijaamyaham.h) with all my powers to restore Faith.
Whenever there is a decline of righteousness, and the rise of unrighteousness, then I re-incarnate myself to teach dharma.
Whenever there is a serious tension in life, when a sort of all-pervasive materialism invades the hearts of human souls - an answering manifestation of wisdom and righteousness is essential to restore equilibrium. The Supreme, though unborn and undying, becomes manifest in human embodiment to overthrow the forces of ignorance and selfishness.
The Lord finally uttered the essence of His teachings in the following verse (18/66) :
"O Arjun! Forsake all your personal beliefs and surrender to me. Do not lament, for I shall deliver you from all sins."