It means to do the required calculations, in order to get a single number as a result. (Or some other data type, which is not numeric - in computer science, date expressions and text expressions are also used, among other things.)
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34 is the evaluation of 34
a*b*c
evaluation, evaluate.
As it stands (because the rest of the expression may be missing) the evaluation to the constant value of -3 will be the result.
A term often used is "evaluate".