What buffering allows you to do is perform actions more quickly by inputting the action (buffering it) shortly before another action is complete. In more technical terms, if you input an action within 9 frames before your first actionable frame after the action is complete, and the action will take place on the first actionable frame, as opposed to you having to wait to input your option until the first actionable frame. The main applications of buffering are to get follow-ups out of throws (buffering a jump or other follow-up) and also to act immediately out of really any action, such as an attack, by buffering a shield, roll, jump, or attack. You can also buffer a spot dodge into another spot dodge to minimize vulnerability frames between consecutive spot dodges.
Performed by inputting an action within 9 frames of the ending of the action you are currently in (generally you mash whatever action you wish to perform).