An expression is a formal construct in mathematics and computer languages. A very general definition of an expression is as follows:

Expression : a (complex) formula that returns a value.

Some examples of expressions: 

       1 + 2                   with result: 3

       'de groene' + 'boom'    with result: 'de groeneboom'

       x * x + 2 * y           result depends on the values of the variables x and y

For more details on the meaning and use of expressions see e.g wikipedia .