The inputs in the left show the inputs for Neck Breaker while Ibuki is performing her 5 active frame crouching medium punch.
A Buffer is a tactic in Street Fighter games where the inputs for attacks are done while another action is active.
Description
In Street Fighter games, most actions do not let players move until said action is completed, such as Ibuki's Reverse Spin Kick, that takes 30 overall frames to finish the entire attack. Street Fighter, however, has a mechanic that allows certain attacks to be canceled. Most characters in the roster have attacks that can be canceled into special attacks, such as Ibuki's crouching medium punch. By buffering an option such as Neckbreaker if her preceding attack hits she will continue to perform Neckbreaker for a 2-hit combo. If the preceding attack does not connect, then she just finishes her normal attack as usual without any change.
Tactics
In this example, Ibuki uses her crouching medium punch outside of it's hitting range, However Xiaoyu's own attack brings her hurtbox into Ibuki's active hitbox of the medium punch. As Ibuki buffered Neckbreaker and the attack connected Ibuki proceeds to beak Xiaoyu's neck because of the buffer.
Buffering allows players better usage of their special attacks. By spacing her attack correctly Ibuki can use her crouching medium punch just at whiffing range and take advantage of it's 5-frame active hitbox, and if they extend their hurtbox into her active extended fist she will do a neckbreaker, an attack that is unsafe if blocked.
Some specific games such as Street Fighter X Tekken have a universal buffer into launcher. A tactic some characters are good at is to buffer their launcher from their light attacks (labeled "Anti Lars Tech" by it's popularizer, due to it's effectiveness against the character and certain others). Characters such as the aforementioned Lars have a noticeable degree of hurtbox extension during their long pokes, in which Anti Lars Tech catches the hurtbox and the following launcher will allow for a full punish.
Buffers are vital for Option Selects. After the initial attack a secondary attack that will be nullified by Screen Freeze if the initial attack is blocked or hits has to be quickly inputed so that if no Screen Freeze occurs the buffered attack comes out.