numbers next to the captured chess pieces


These numbers are how much material value advantage one player has over the other.
Pawn = 1
Bishop/Knight = 3
Rook = 5
Queen = 9
So if you see a +1, then that means someone is up about one pawn. It could mean that one player has one extra pawn on the board. Or perhaps one player traded two bishops (6 material value) for a rook (5 material value), for example.
https://www.chess.com/terms/chess-piece-value