It's the engine's (computers) evaluation of the position. A positive value indicates an advantage for white, a negative value indicates an advantage for black.
eg.
+1 means that white has an advantage equivalent to 1 extra pawn of material.
-1 means that black has the same advantage.
If white gave away a free Knight in the opening, for example, you'd notice the evaluation shift from approximately 0 to -3, which means black's advantage is now equivalent to three pawns worth of material - which it would be as the Knight is worth 3 points.
Typically 1 pawn = 1 point of material, a Knight or Bishop = 3, a Rook = 5 and a Queen = 9.
Sometimes the computer will evaluate a piece to be worth more or less than its typical value dependent on the position. eg. towards the end of a game, if you both have 1 king and 1 queen each, and one side gives away a free queen in that position, rather than showing + or - 9 for the other side, it might re-evaluate the position as something like this "#5" this means it's calculated a forced checkmate line in a minimum of 5 moves. Aside from + or -, you may see # - this symbol generally signifies a "forced checkmate in X moves".
Hello,
I'm a beginner to chess. Can anyone tell me what the numbers and bars mean next to each move?
Many thanks
Brisbane29