A nested list is a list that contains another list (i.e.: a list of lists). It is also referred to as a multi-diminsional array. For example, a 2 dimensional array is used below:
nested_list = [['blue', 'green'], ['red', 'black'], ['blue', 'white']]
print len(nested_list)
# prints 3
print nested_list[1]
# prints ['red', 'black']
print nested_list[1][0]
# prints red
To go through every element in this list, use a nested for loop.
>>> nested_list = [['blue', 'green'], ['red', 'black'], ['blue', 'white']]
>>> for inner in nested_list:
... for value in inner:
... print value
...
blue
green
red
black
blue
white