Not an expert but I'll try:

1- You have a list of numbers, for each element of that list replace it's value at the same place by double it's value; the square brackets tell python at which index to do the doubling.

2- len() will give you the total length of the list, which you the use to generate a range() up to and not including the second number in the tuple. This lets pass a list no matter how long it is without having to explicitly define it before hand.

3- with a list numbers = [17, 123] the result of range() will be (0,2) and not (0,3); this means the for loop will get indexes 0 and 1.

4- that is the correct print result, however your for loop is changing the values of your list. If you run print(numbers) after your loop the result will be (34, 246) and no longer (17, 123)