Frame data is knowing how many startup, active, and recovery frames a move have, and also, how many frames of hitstun, or blockstun a move has, and also other things in general like, iframes and whatnot.
The general idea is, for example, a move has 15 frames of recovery, but deals 10 frames of blockstun, then, the enemy will recover 5 frames earlier than you, so the move is -5... That is the basic idea.
That's still a thing that makes me shake my head to both SoulCalibur & Tekken. What good comes out of hiding the frame data? Moronic move by Namco & Harada.