Let's say we have a function "y" = f("x"). f is short for function, so this equation means "y is a function of x". This tells us that how high y is on the vertical axis depends on what x (the horizontal axis) is at that time. For example, with the equation "y" = "x"², we know that if "x" is 1, then "y" will be 1; if "x" is 3, then "y" will be 9; if "x" is 20, then "y" will be 400.