So, what should I be looking at in particular for the frame data of combo strings? Like does start up/active frames matter here? Should I only look at the block/hit advantage for the combo strings vs. looking at everything for all the "basic attacks" (normals)?
The game is based around 60 frames per second so 1 frame is 1/60th of a second (to give you an idea of how frames work).
-Startup frames tell you after how many frames your attack will get a hitbox. A move with 6 startup frames will take 0.1 seconds (6/60 frames) to come out. Startup is most important when it comes to punishing attacks. If your opponent does a move that is -7 on block, they cannot block for 7 frames. If you use a 6-frame attack, you can hit them before they can block and thus get a combo. In this case, an 8-frame attack would not work.
-Active frames tells you for how many frames the move has a hitbox. I wouldn't worry about this too much, you only need these frames if you want to do some whacky things on block which I can go into if you want.
-Recovery frames tell you how many frames it takes for you to be able to move again if you miss the attack. So a move with 8 startup, 6 active and 10 recovery frames has an initial 8+6=14 frames to be done with the attack itself, and takes 10 frames for you to move again afterwards.
-Advantage on block tells you about your situation after your opponent blocks one of your moves. If this number is negative, say -10, you cannot move for 10 frames after you have done the move. During this time your opponent can punish your attack if they have a move that is 9 frames or faster (a 10-frame move would not be able to punish a -10 move btw).
If this number is positive, you can move earlier than your opponent. So if your move is +10 on block (listed in the frame data as just '10' without a +), you can move 10 frames earlier than your opponent can. If you do a move that is 9 frames or lower, your attack will jail and the opponent has no choice but to block it.
Generally speaking, the fastest attacks in the game are 6 frames so a move that is -6 on block or higher ('higher' being -5,-4, etc.) it is called 'safe on block', because it cannot be punished by anybody assuming that the fastest attack in the game is 6 frames.
Moves that are + on block (or 0 on block) are generally ones you should keep in mind. Be careful aboutbusong moves that are like -10 or lower, because these can be punished fairly easily.
- Advantage on hit tells you how long your opponent has to wait before they can move, if they get hit by your attack. Typically pay attention to when this value is like 15 frames or higher on moves tjat knock your opponent down or restand them.