Pressure and Turns
Frame Advantage creates the concept of Pressure and Turns in Fighting Games. Being at frame advantage after landing a move is like having it be your "turn", being at frame disadvantage after landing a move is like handing your turn over to your opponent. If you have frame advantage, your attacks come out first, which means using your fastest move for that range will almost always beat your opponent's attempt to do the same. So in general, when it's your turn, you can do whatever you want, and when it's your opponent's turn, they can do whatever they want.
So the most obvious way this works is with a blockstring. You have a sequence of moves that are plus on block that eventually become minus or the combo chain ends. If your opponent tries to press a button during this sequence, they'll get frame trapped and you get a combo or stray hit. You can follow your block string through to the end, and if your opponent blocks the whole thing, your turn ends and the game returns to neutral (if you use a move that's minus and your opponent is close, then it becomes the opponent's turn instead of returning to neutral).
So that's nice and all, but if your opponent just waits out your turn, you don't get any advantage off of it. What makes frame advantage actually scary is the option to do things other than just continue your block string, such as dashing in to restart your pressure, or suddenly going for a throw or overhead to break the opponent's defense. If the opponent blocks complacently, these can be harder to react to. In some games in some cases, you can spend meter to reset block strings without a gap, or turn unsafe moves into blockstrings. Be aware of your options and mix it up, while paying attention to your opponent's reaction.
If you do a move that's minus, you're handing your turn over to your opponent, which means they get the same options versus you. They could just attack back, pressuring you, or take advantage to get a mixup or throw in on you.
Even if it's your turn, that doesn't mean your opponent is completely without options however. If they have an invincible reversal move, they can use this in the middle of your block string to interrupt you (assuming there's a gap, and it's not a true blockstring). Some games also offer other options such as Push Block, Guard Cancels (like Alpha Counter or Dead Angle Attack), and Parries to beat blockstrings. Pushblock can help return the game to neutral (or push the opponent off right before they do a move with a ton of recovery, leaving them open to a whiff punish), Guard Cancels can beat blockstrings much like a reversal, but they don't care whether it's a true blockstring or not. Parries can interrupt and punish a block-string outright, requiring attackers to mix up their blockstring options versus the parry zones or sometimes try to bait out the parry attempt.
Push block is the safest of these, generally being completely safe, or having extremely high commitment ways to beat it. Guard Cancels frequently give the attacker a chance to react and stop attacking, but since they can be canceled from block, they're in a good position to interrupt the next attack if the opponent is committed, Parries and Invincible Reversals are pure reads, but can also give guaranteed punishes if baited.