An expression is a series of integers, operations, and in some cases symbols that are piled together to create a value of some kind. Expressions do not have equal signs or inequalities. Expressions help us to better understand the world around us by stating problems that we can explore and mathematically determine the value or truth of.
Expressions are basically mathematical phrases. A way to describe the world around us or math based situations that we come across. For example, what if you had a 16 inch piece of wood that is 2 inches too big for a corner piece of the deck. To express this mimetically we could write the expression 16 inches - 2 inches is our desired size or 16 - 2.