To hit confirm a move is to use a combo consisting of safe-on-block moves, to check whether you are successfully hitting an opponent or not before comboing into an unsafe move.
The basics are to first find a string of moves that combo on hit, but are safe on block. Then, if a move connects, you know you can cancel the next hit in the string safely. This is because in most games, you need to cancel during hit freeze, so you do not have time to see whether a move is hitting before you cancel it. Therefore you must use earlier moves in the combo to tell if you are actually hitting before you cancel into something unsafe, thus confirming.
There are a few other ways to hit confirm as well. Some moves let you cancel them late, after the hit freeze, and have an especially late window to cancel, or especially long hitstop, giving you more time to visually see if you're hitting or not, allowing you to confirm off one move, instead of two or more.
You can also poke with buffered special cancel just outside where your opponent is currently standing. This way if your opponent walks forwards, they'll run into your attack, which then cancels into the special on hit, and if they don't walk forwards, your attack will whiff and you won't do the unsafe special move. A variation of this trick is whiff punishing your opponent's outstretched limb. If you miss, again, you won't cancel into something unsafe.
Taking advantage of Counter Hits with Frame Traps can help extend block strings, allowing you to confirm off strings that normally don't combo.