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)
