Strings in python

Can someone please explain lines 7 and 8 to me in this code the original question wants me to:

  1. Write a function called contains that takes two arguments, big_string and little_string and returns True if big_string contains little_string .

For example contains("watermelon", "melon") should return True and contains("watermelon", "berry") should return False

  1. Write a function called common_letters that takes two arguments, string_one and string_two and then returns a list with all of the letters they have in common.

here is the answer but I don’t understand the lines I’m asking help for:

Line 7 is a conditional designed to check if the letter being iterated on in the for loop (variable letter) also exists in string_two, and also to check that the letter does not already exist in the common list.

So, for each letter in the first string, you’re checking to see if the letter also appears in the second string, and if it does, you’re checking to make sure you haven’t already identified this one as a common letter.

Line 8 is responsible for adding letter to the common list: Since it’s already passed the aforementioned conditions, it’s free to be added to our resultant list which is returned at the end of the function.