It means you have two variables ... an independent variable "x" and a dependent variable "f(x)" or sometimes labeled "y"
The dependent variable f(x) gets input from the independent variable that results in output from the dependent variable.
Think of f(x) as an input and output machine. It accepts x and out comes f(x)
For example, let the machine or f(x) follow the rule "3x". In comes an independent variable x=4 into the machine and then f(x) spits out 3(4) or 12=y.
Hope that helps!