I have a question regarding for loops and the difference of using for example array[x] and just x in Python

Given:

array= [1,3,5]

def function(array): array.sort() sum = 0 for x in array: if array[x] > sum + 1: return sum + 1 sum += array[x] return sum + 1 

Why is it that I can't access the array using array[x] but I can by just saying if x>sum+1

What is the difference of using array[x] and just x in that if statement inside that loop?