Question 144225
x is one of many "unknowns" that we use to represent a quantity that varies, or to indicate a number that we would like to find.

Consider:
x+1=3

What plus one makes three? In this case x stands for a number that solves this equation.

Now consider:
f(x)=x^2-1

What this equation describes is the relationship between one unknown that can change to give another quantity that depends on it. So if we take f(2) we will find 2^2-1=3. So an x value at 2 gives a y value of 3. You could say x serves to describe what happens to the other quantity.