It means to solve, or simplify the expression.
Let's give two examples.
1) 5 + 2²
5 + 4
9 <- We are trying to simplify/solve the expression to as far as we can.
9 cannot be simplified, so the expression has been evaluated.
2) 4x - 5x + 9y
-x + 9y <- Can not be simplified any further so the expression has been evaluated via. combining like terms.