Here's a bit of an explanation of the control flow and what's going on with Python's indexing, hope it helps:
s = "wxyabcd"
myString = s[0] # 'w'
longest = s[0] # 'w' again, for collecting other chars
for i in range(1, len(s)): # from 1 to 7, exclusive of 7, so 2nd index to last
if s[i] >= myString[-1]: # compare the chars, e.g. z > a, so x and y => True
myString += s[i] # concatenate on to 'w'
if len(myString) > len(longest): # evident?
longest = myString # reassign longest to myString
else:
myString = s[i] # reassign myString to where you are in s.
print longest