Lets break the code down a little bit.
n = [3, 5, 7] # This is the list that will be iterated over
result = 0 # This keeps track of the total amount
This is the more important part:
for x in range(len(numbers)):
will give you the amount of items in list n. In this case the amount of items would be 3.
(with 3 being the result from len) returns a list with numbers ranging from 0 to 2, so
[0, 1, 2]
Oversimplified, your statement says:
for x in [0,1,2]
The x is a variable for the index of the list you are iterating over. It can have any name. You can use this variable to access any item of the list. numbers would output 3, etc.
result += numbers[x] # add the number to the sum (numbers, numbers and numbers)
Finally, we print the sum: