"Buffering" means performing an action just before another action ends, so that the action comes out on the first frame. For example, if you side-B (flame choke), and press jab during the final 10 frames of the side-B, your jab will come out on the very first frame that the side-B ends. Same with D-tilt, F-tilt, and dash attack. (Jab is pressing A, Dtilt (down tilt) is holding down and pressing A, Ftilt (forward tilt) is holding < or > and pressing A, and dash attack is pressing A while running).
Buffering is important because it means that your timing doesn't need to be perfect. Instead of a 1-frame window, you have a 10-frame window. Hitting your opponent with jab/Dtilt/Ftilt/dash attack after a side-B requires strict timing, but buffering makes it much easier.