A variable is a letter, such as x or y, that represents a number. Another way of thinking about it is that a variable is "code" for a number. Mathematicians can spend hours at a party talking to one another in variables and no one will have the slightest clue what they are talking about.
The most common variable used is x. When we have multiple variables, we may use x, y and z. However, any letter will work, as long as we are consistent with it. We tend to use x, y and z because we feel bad for them having to hang out at the back of the alphabet all the time. This is their time to shine.