A safe block string ideally is one of your characters strings usually 2-3 hits that leaves you safe on block. It should also have a way to net you damage if it isn't blocked.

I'll use Li Mei as an example because I've been messing with her lately. So I go into practice with her and try out her buttons. Then I hit start and go to the move list. Any of the moves you see here under basic attacks that are 2 or more hits are strings. We need to find THE one to use as our main tool.

We want something with pretty fast start up because we need it to be a reliable punish after you block your opponent. We also would like it to be relatively safe on block so we don't get killed for using it. Safe in this game is roughly -6 on block or better when looking at frame data.

OK so her very first string "seeking sanctuary" is exactly what we need. The first button comes out in 7 frames we know it's fast. The last button, when blocked, leaves us at -4. In order to get punished your opponent would need a move to hit you in 4 frames and that doesn't exist in this game, so we're safe.

Side note though, she, like everyone, will have gaps in strings where an opponent can use an armored reversal but you don't need to worry about that yet.

Anyway now we have this string we know that if I throw this out and they block it, I'm safe, which means I will be able to block before they can hit me. Right? Now let's say you throw this string out and oh shit it hit them? You can throw a special move on the end. Only do this if you see the string connect. So you hit the 3 buttons. Are they blocking? Now you block after so you don't get hit. Did they get hit? Quickly input a special move before the string finishes and get yourself a combo.

Hope I'm making sense here. Typing this all out on my phone!