Hit Confirm is the act of doing the first hit of a combo, and only continuing the combo if the first hit actually hit the opponent - on block you stop your combo or do a safer move instead. You are confirming the first attack of a combo actually hit the opponent before continuing. This sounds simple, but can be difficult to do depending on how much time the first hit affords you to react and perform the second hit.
Types of Hit Confirms
Some combos only work if other conditions are met such as Counter Hit, crouching hit, air hit, being close, or being far. Because of this, hit confirming based on hit and block may not be enough and players will need to be able to differentiate how the opponent was hit.
Hit Confirming Techniques
Instead of just reacting to the result of one attack before doing the rest of a combo, try hit confirming after the first 2 attacks of a combo instead. This will give you additional time to react and hit confirm correctly.
Do note that this might not always be possible with certain hit confirms since it relies on Counter Hit or other properties.
Some players have problems hit confirming normals > specials or normals > supers because executing the second move is a more complex motion and thus takes longer to complete. A common technique is to begin doing the motion before the attack even touches the opponent, then once you can confirm that the attack hits, then simply press the button to execute the special/super.
This technique of doing the motion beforehand is called buffering and can help with hit confirms and combos in general!
Try delaying the second hit as much as possible and see just how much time you have to react to the hit. This will give you an idea of how much time you have to work with. Make sure the amount of time is reasonable for you to react to, since some combos are simply not hit confirm-able.
To help with this, go to training mode and set the training dummy to Guard > Random to help you practice confirming hit vs block. You can Set Counter Hit > Random to practice counter hit specific hit confirms too!