Chromatic Aberration is the effect of light getting broken into spectral colours when entering the glass of a lens by refracting slightly differently depending on wavelength. It can help your CG look a teensy bit more realistic. (Unless you overuse it... Hrm...)

While I prefer to use Magic Bullet Looks for subtle Chromatic Aberration in After Effects, not all of us can use it, because not all of us have it.

Using Shift Channels and Optics Compensations I show you a technique you can use to make it from scratch, and it gives a bit more control.