It slices
x[startAt:endBefore:skip]
if you use skip = 2, every other element the list beginning at startAt and ending at endBefore will be selected. [Remember: indices live BETWEEN list elements]
To see this, enter
x = range(100)
at the Python prompt. Then try these things
x[::2]
x[::3]
x[10:40:6]
and see what happens.