The number usually relates to an advantage measured in Pawns. Everything else in a position might be equal but you might have a Rook on the 7th rank, something an engine MIGHT regard as an advantage equivalent to being 2.5 pawns up. The number displayed might then be 2.5 to signify this.
If we take the same position and say that you have that rook on the 7th, but you sacrificed a pawn in order to get it, putting you a pawn behind, the engine might evaluate the advantage as 1.5 pawns up. The given score is thus an aggregate of not just the material imbalance but all the positional factors which that engine can evaluate.
There is another detail - most engines assign negative values for an advantage to black and positive for white. So a score of -2.5 means that Black has an advantage equivalent to being 2.5 pawns up. A score of 2.5 means the same advantage to White.
A score of 0.00 signifies that the engine thinks the position is dead equal. We could use a shorthand symbol '=' to indicate this 'balance'. An engine might use a '+=' to signify that the advantage has swung in Whites favour, and '=+' the same for Black. Fritz does this in it's annotations (if i remember right.) A winning advantage is shown as a heavier inbalance such as '++=' or '=++'.
Other numbers in engine analysis usually relate to the depth (how many moves deep it's looked into a position) or the time it's taken to arrive at an evaluation.