Hitconfirming is the ability to react to cues to know if a move hits or is blocked. This is useful because most moves have very different situations on block vs on hit, and some allow guaranteed followups with sharp hit confirming.
The most consistent cues are white block sparks, guard animation and sound. However, you can also watch your opponent closely for ducking, stepping and attack startups during the startup of your own attack, to supplement normal hitconfirming.
This is particularly helpful to confirm a counter hit or confirm strings with a tight follow-up, which may be completely impossible to do on block sparks.
In-game examples: With you at +1, if you and your opponent both attack out of blockstun with i10 jabs, their attack animation could give you an extra 9 frames to react and hitconfirm before your jab interrupted it.
If at +0, you sidestep for 11 frames and use an i13 DF1, and your opponent crouch jabbed immediately, you have a full 24 frames to react to the whiffed crouch jab before your move impacts them. If your DF1 has a natural combo follow-up with a 6 frame input window after impact, this gives you a full 30 frames to confirm it. To attempt it using block sparks would only give 6 frames.