The parameter given here is 3, although it doesn't have to be a number. You might have noticed that this looks the same a the function declaration at the top. This is true. Basically, this line restates the declaration and tells the compiler that the following will be that function we mentioned earlier. Notice that you don't use the semicolon here.
Functions return a value.
That is the point of functions. In this case, we wanted to do some simple math and return b. Sometimes you just want a function to perform a task without returning a value. Well, it's a start. There is so much more.