Explain what the variable represents in an addition or subtraction expression

The variable represents something that is unknown. If I want to know what to subtract from 13 to get 10, I would say 13-X=10

In this problem the Variable is X, but you can use any letter or symbol, I would not use "S", "O", or "l" to represent a variable, because they can look like 5, 0, and 1.

A variable always represents an unknown quantity.

We have to have some method to solve for an unknown, and we must have something to hold the place of the unknown, so we use a variable.