What does i in range(len(nums)-2): do? in Python

Question

I understand list iteration and range and len() function but I'm not getting what the -2 does exactly..

this was a problem on codingBat

Given an array of ints, return True if the sequence of numbers 1, 2, 3 appears in the array somewhere.

    array123([1, 1, 2, 3, 1]) → True
array123([1, 1, 2, 4, 1]) → False
array123([1, 1, 2, 1, 2, 3]) → True

Solution:

    def array123(nums):
  # Note: iterate with length-2, so can use i+1 and i+2 in the loop
  **for i in range(len(nums)-2):**
    if nums[i]==1 and nums[i+1]==2 and nums[i+2]==3:
      return True
  return False

Answer 1

Assuming nums is a list or an array, the len() built in Python function will get the length of items in an array and then we iterate in a for loop starting at 0 and continuing to add 1 to the variable "i" until it is equal to the length of "nums" minus two and we stop once it reaches that threshold.