It's an indication of how much that person is ahead in the game, in terms of material, using the usual "weight" of pieces: so if you were a pawn up, it would be +1; if you were a Queen up, it would be +9 (I think).
+ sign next to captured pieces.
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Apologies for the newbie question or if it's been asked before but what does the +1,+2 etc sign next to captured pieces mean? Been bugging me for a while now.