FXAA or Fast approximate anti-aliasing is an anti-aliasing algorithm created by Timothy Lottes at NVIDIA. The main advantage of this technique over conventional spatial anti-aliasing is that it does not require large amounts of computing power

In layman’s terms, FXAA is applied to your final rendered image and works based on pixel data, not geometry. GPU’s are particularly fast at executing these shader algorithms in parallel, thus it’s very quick to render.

If you’ve ever had a case of crunchy edges (aka jaggies, stairstepping, aliasing) in After Effects, FXAA will kill those for you. These issues are particularly common when doing motion-trails or motion smears using the Echo effect or Cartoon Moblur.