Inputting or holding a command before it can occur. Buffering techniques make consistent gameplay a little easier by essentially making inputs more lenient. Two kinds of buffering are possible in Super Smash Bros. Ultimate.
The first is typical buffering, where certain early inputs are carried forward and executed at the earliest possible moment. The most important global example of this is the buffer period when dash-canceling. Flick the control stick left or right to start your fighter’s ground dash, then quickly return the control stick to neutral and input any other attack. (The simplest example is to flick the stick, then try a standing neutral attack, just to see how it works.) Correctly done, you’ll be able to feel that you input the command early, but you’ll see the fighter interrupt the dash at the halfway point with the new command. Any desired action buffers forward and cancels the dash except shielding. This helps ensure your execution is fluid and consistent during aggressive ground movement. This kind of buffer isn’t universal to movement actions; it just works for canceling dashes. Some fighters have special follow-up actions or mechanics that can also be buffered (most fighters do their full neutral attack combo if you simply hold down the Attack button, for instance), but this is the biggest example of buffering involving early inputs carried forward.
The second is hold-buffering. This is possible anytime your fighter is in a locked state, busy performing some other action (attacking, shielding, dodging or rolling, anything that prevents them from immediately accepting your input). All you have to do is hold down the next desired action. That’s it. Performed correctly, your fighter will carry out the input you’re holding as soon as they can.
Naturally, implementing this intelligently can go a long way toward improving your results.
The easiest way to implement this is after blocking opponents’ moves. Either release Shield and hold down Attack (to come out of blocking ASAP with a quick combo), or keep holding Shield and start holding Attack (to cancel the shield animation with a shield grab). Many fighters have a powerful up special that can work as a kind of reversal, or a launching up tilt or up smash attack; you can block, then hold down the command for one of those to counterattack out-of-shield as fast as possible!
Another example is to help time combos. As your fighter is still carrying out one attack, you can start holding the next tilt/smash/short hop input, or the Jump button to chase them up instantly.
Defensively, in midair, when stuck in hitstun, currently eating attacks, or when sent tumbling, trying to regain control, hold the Shield button to air-dodge (neutral or directed) at the earliest possible moment. On the ground when being hit, hold Shield to block whenever possible. There’s a limit to this trick: you can’t hold Shield and a direction to spot-dodge or dodge-roll on the ground at the first possible moment. You have to buffer raising the shield, then manually time trying to sidestep or roll.