A Lagrangian point is basically a spot in space that has gravitational fields acting on it equally in such a way that there is no net force. Basically it is a place that you can shoot the bird that it will get stuck in two overlapping gravitational fields. You will most certainly get this without even trying while collecting all the stars.
A good spot to earn this is Level 1-10. Just fire the bird as slow as you can right down the middle and it should get stuck. It will take a few tries to get the speed slow enough and the angle perfect. Thanks to I Spiritus I from TA for this method.