• got stuck on sort a dict

Question related to mission The Most Wanted Letter

import re

def checkio(text):

    text = text.lower()
    list1 = re.findall(r'[a-zA-Z]',text)
    set1 = set(list1)
    times = []
    for i in set1:
    d = dict(zip(set1,times))
    d1 = sorted(d.items(),key = lambda x: (-x[1], x[0])) 
    return d1[0][0]

I finally solved this after i saw the hints. but i can't figure out the code to sort a dict here :

sorted(d.items(),key = lambda x: (-x[1], x[0]))

could anyone kindly send some explanations or guides to me?