What does B mean in fighting games?

Excuse me, I'm a bit puzzled about a term in fighting games. Could you possibly explain what does the letter "B" stand for in this context? I've noticed it mentioned quite often in discussions about game strategies and techniques, but I'm not quite sure what it refers to. Is it a specific move, a type of combo, or perhaps a category of characters? I'd really appreciate it if you could clarify this for me. I'm trying to improve my gameplay and understand the lingo better. Thanks in advance for your help!