Let's use the add function in some code and run through it in detail.

We define the function first, giving it a name (add) and some code.

We don't have a value to assign to totalNumber yet, we have a function call. So at this point we 'call' the named function. To 'call' a function, we change the current instruction we’re processing from line 4 to line 1.

    def add(a, b)

Then, at line 1 in our mind we create variables for each input. This gives us two variables, a and b with the values we used to call the function.

Then we go to the next line of code, line 2

    return a + b

In this case we perform the substitution to get

    return 4 + 13

Which is the plus instruction, so we replace the 4 + 13 with the result.

    return 17

Now the function’s code block has finished so we return to place where the function was called, line 4.

    totalNumber = add(4, 13)

We’re still in the middle of performing the substitutions, so we can replace the add(4,13) with the resulting value returned from the function, 17

    totalNumber = 17

Now we have a value, we can perform the assignment. Then we proceed to the next line, which prints the value.