What does 'for i in range(n)' in python mean ?

If you just want to do something n amount of times and don't need the iterator and you can just live with the extra ivariable.
_ is just another variable.
_is assigned the last result that returned in an interactive python session:

>>> 1+2
3
>>> _
3

or

>>> for _ in xrange(10): pass...
>>> _
9
>>> 1+2
3
>>> _
9