Explanation: 
An expression in math is composed of numbers, arithmetic operators, and it also can have variables. 
An expression can have the following components:
Coefficient
Variable terms 
Constant terms
Arithmetic operators
Where the coefficient is the number that accompanies the variables, the variable terms are the ones that have a letter, constant terms consists of only a number without variable, and the arithmetic operators can be +, - /, x, etc. 
For example, an expression can be:

or

These expressions have numbers and arithmetic operators. 
The expressions can also have variables, for example:

Or it can have more terms:

It is important to state that expressions don't have an equal symbol (=), in that case, it is called an equation. 
Answer: An expression in math is composed of numbers, arithmetic operators, and it also can have variables.