F(x) stands for a "function of x"
I.e y=mx , where y depends on what x is. X is the independant variable, and y is the dependant variable. Basically, whatever you get for x, determiens what y will be. That said, F(X) defines x as the independant variable.
For example, if I choose to plug in 5 or x, which is the same as saying x=5, then the function will start with F(5). Of course you must plug in this number to find out what it means. Think of it as a machine, or more speficially a function machine. You put something in, it processes it, and outputs something.
Assuming I use the equation f(x) = x+5 , that says that my independant variable will be modified by adding five to give me the resultant dependant variable. If x0, that will tell me what my y value is when x=o, or my y-intercept.