Buffering is a technique in fighting games where the player performs the incomplete inputs for a special or super move and presses the button to confirm the attack only when he sees specific visual cues, e.g. the opponent walking forward or performing a specific attack. Example: a Ryu player may continuously input quarter circle forward in neutral and press punch to throw a fireball only if he sees the opponent doing a dash.
The advantage of using input buffering lies in shortening the reaction times required to punish a certain move all while being mostly safe because you don't commit to the special attack until the opponent. Some attacks that would be hard to punish by purely reacting to their startup frames may be easily hit through the use of buffering and thereby allow the player to land counter-hit combos.