It simply means that you can do something before the other person. In most situations this just means that if you both do an attack you'll win since yours will come out first. Frame advantage doesn't translate into guaranteed damage, it just gives you an edge in the guessing game.
If you want guaranteed pressure, you need to hit them before their blockstun ends. You don't get any instant rewards from this since it's guaranteed to be blocked, but you don't give your opponent a chance to get away, which can frustrate them.
If you want a standard counter-hit setup, you need to hit shortly after their blockstun ends, but before their fastest attack can come out. If you have +4, and their fastest attack has 3 frames of startup, you need to hit them on the fifth or sixth frame after you recover, which means either using a move with four or five frames of startup or waiting a little bit and using a faster one. It's generally good to use moves that cover common escapes in this situation, such as jump, backdash, and reversals if possible.
If you have a lot of frame advantage you can also use it to safely set something up or for positioning.
While those are basically the only ways to directly take advantage of frame advantage you can also use whatever mixups you have for those situations to indirectly take advantage of it, for example if your opponent knows you have frame advantage and doesn't want to get counter-hit they are probably going to block which means you can grab them. Anything further than this is character/situation dependent. It's like if you're playing rock, paper, scissors, frame advantages means you win if you both pick rock. While this doesn't improve your scissors, it does indirectly help since your opponent doesn't want to play rock as much.