It's basically an indicator of how safe the move is on shield, the more negative the number, the more unsafe it is. "Frame advantage" is the technical term I think.
If a move is -10 on shield, it means that the shielder can act 10 frames before the attacker can. A move listed as +10 on shield means that the attacker can act 10 frames sooner than the shielder. I don't know how exactly the math works, but it's largely a result of move damage, endlag/landing lag, and shield drop frames
It's a smidge more complicated than that though, as you also need to account for the move's range, the opponent's OoS options, and human reaction time in order to get a better sense of shield safety