Hello there!
I want you to think of functions as machines! Let's start with one function, and we will call it f. if we were to plug in x to the machine, x would be an input. Let's say we get 1 everyone we plug in x to the f machine. This would mean the 1 is the output! f(x)=1
Now let's talk about composite functions! Let's say we have TWO machines, f and g. If we plug in x to the f machine, then we would get 1. If we plugged that 1 that we got as an output from the f machine into the g machine, then we would have g(1)=5. THIS is how a composite function works. g(1) is actually g(f(x)) because f(x)=1.
I REALLY hope this analogy helps you out! I apologize if it doesn't, however I've noticed that many people have had success in thinking about functions in this way!
If you need more elaboration, feel free to message me!
Best wishes :)