What is a mathematical expression?

Answer 1

Any "expression" that represents a numeric value. Example: 2+2=4 or x+7=10. Actually the examples above are equations, not expressions. Expressions do not have = signs. 7a and 4x are examples.

Answer 2

A mathematical expression is a series of mathematical terms with no equals sign, e.g
3x-2y+4
is a mathematical expression.
You can evaluate a mathematical expression like this,
3x+2x
becomes 5x, so what you are effectively doing is simplifying the expression. Hope that helps.