mystery string code - help!

[Uncle Ben]

I found this in one of the online cookbooks:

#Raghunath Reddy Peesari 6 years, 3 months ago  # | flag
#There is more simple way. ###

a = 'abcdefghi'
a = a[::-1]

print a

>>> 'ihgfedcba'

As a newbie Pythoner, I understand [] -1]
but would some tell me how '::' does its magic?

Uncle Ben