help explain code in detail please

1 def main(): 2 3 4 5 # Local variables. count = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0] letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' index = 0 frequent = 0 6 7 8 9 10 # Get input. user_string = input('Enter a string: ') for ch in user_string: ch - ch.upper() 11 12 13 14 15 # Determine which letter this character is. index = letters.find(ch) if index >= B: 16 17 18 19 count[index] = count[index] + 1 20 21 22 23 for i in range(len(count)): if count[i] > count(frequent]: frequent - i 24 25 26 print('The character that appears most frequently ' in the string is ', letters [frequent]) 27 28 29 30 # Call main main() 31