What does it mean to evaluate an expression?

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.

Evaluate an expression means to simplifying an expression down to a numerical value

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