What's the name and how it works the black&white bar?

+1 is an advantage worth one pawn for white; -1 is an advantage for black worth one pawn. So +4 would be an advantage for white worth 4 pawns give or take.
In practical terms, if material is equal and the engine is showing an advantage of much over a pawn for one side there is probably some tactical thing you are missing. An advantage of less than a pawn is likely to be some positional or at least activity related thing you are missing.