At a Glance - Variables
A variable is a letter, like x or y, that represents an unknown 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're talking about.
The most common variable used is x. When we have multiple variables, we might use x, y, or even z. Really, any letter will work, as long as we're 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.