X means everything in algebra. It is a variable, which means that its meaning changes based on context. For example, x could be time, but it could also be cars. x is what all Mathematicians chase after and it is also the greatest tool at their disposal. x is generally called an independent variable because its value determines the output of a function. y, on the other hand, is the result of fooling around with x. It's a dependent variable, which means that y changes when x changes.
Take, for instance, the following function: y = x + 3
Let's set x = 3. y = (3) + 3 y = 6
As you can see, what we input as x determines what y outputs.
