f is a function. It takes inputs and then produces an output. The notation f(x) is saying more specifically that the function is taking an input x and producing the output f(x). For example, if you look at f(x)=x2, you can think that f is the "squaring function", with input x and outputs x2. You can think that "f" is like the name of the function. f1(x)=x2 f2(x)=6x+3 Above, you know which function I'm talking about if I say f2. But often, people will just call it as the function f2(x)