A simple expression is a series of terms containing variables or constants, or both, as operands and utilising arithmetic operators normally restricted to the arithmetic operations: add, subtract, multiply, divide and possibly parentheses or brackets.
Homework will usually consist of evaluating or simplifying, or both, such expressions, employing rules of order of operations (PEMDAS, BODMAS, etc.) and the distributive, associative, cumulative and other properties of numbers.