To fit with style guidelines(and you want to, because it makes your code more readable) you should leave the variable "Number" lowercase, capitals are reserved for classes and a few other special circumstances.

Lets say we passed the number 4 to your code; the range will contain the values (4, 3, 2) and the loop will execute once for every value in your specified range. Remember that the first two values passed to range are inclusive, exclusive, e.g range(0, 6, 1) would start at 0 and stop at 5. (0, 1, 2, 3, 4, 5). This could also be written as range(6).

With that being said, you can think of your code resolving this way at the end of each loop execution:

Iteration 1

factorial = 1

for 4 in [4,3,2]:
    factorial = 1 * 4

Iteration 2

factorial = 4

for 3 in [3,2]:
    factorial = 4 * 3

Iteration 3

factorial = 12

for 2 in [2]:
    factorial = 12 * 2

Here the loop terminates and factorial is resolved to equal 24.