Which course is this for? I feel like I may need a little more information, but the simplest explanation I can give given the question is:
f(x) is the value of a function f evaluated at the value x. The formal definition of a function is a particular set of rules that associates a particular value with another. For an analogy: you can think of f as a machine that takes in an input, transforms it according to a certain set of rules, and produces an output. In this analogy, x would be the input and f(x) would be the output.