so f(x)=x+10 lets say
so if you put something into the function lets say 5 then the function will do something to it and give you an answer so if you put in 5 for x you would get 5+10 or 15
a function of a function
lets say
f(x)=2x-9
g(x)=7x+2
(g(x) is just another function represented by g instead of f)
so if you wanted to put g(x) as x into f(x) and put x in g(x) to 3
then f(g(x))=2(7x+2)-9 or f(g(3))=2(7*3+2)-9=2(23)-9=37
hope this helps