A lot of responses here so I'll suggest a different approach. You can watch your python code execute line by line using a debugger. That might help with understanding how it all works.
def my_sum(list):
result = 0
for number in list:
result += number
return result
my_list = [1, 2, 3, 4, 5]
list_sum = my_sum(my_list)
print(list_sum) # Prints 15
If you run the above code line by line in a debugger, you'll see that when it gets to list_sum = my_sum(my_list) the program will jump into the function my_sum(list) where "list" is a variable holding the value of "my_list". The program continues line by line inside of the function until it hits the return result statement. The program then returns to the line it was at before jumping into the function. "my_sum(my_list)" now has an actual value. It's the value that the return statement provided. The line would now read list_sum = 15 to python.
A debugger shows you which lines get executed in which order and how the variables update and change with each line.
Just a note: python has a built-in sum() function you could use instead of writing your own my_sum() function, but a debugger won't show you how built-in functions work! They're built into the language itself. You'd need to look up Python's documentation to see how they actually function under the hood.