Yada yada hi dharmasya, glanir bhavati bharata
Abhyuthanam adharmasya tadatmaanam srujaamyaham
Paritranaaya saadhunaam vinaashaaya ch dushcritaam
Dharmasanstha panaarthaaya sambhavaami yuge yuge
The simple meaning is :
Whenever dharma declines, O Bharat (Arjun), and the purpose of life is forgotten, I will manifest myself
I am born in every age (yugas) to protect the good, to destroy the evil and reestablish dharma
