[Python] Explaining Python's '__enter__' and '__exit__'

I saw this in someone's code. What does it mean?

def __enter__(self):
return self

def __exit__(self, type, value, tb):
self.stream.close()


Here is the complete code.

from __future__ import with_statement#for python2.5

class a(object):
def __enter__(self):
print 'sss'
return 'sss111'
def __exit__(self ,type, value, traceback):
print 'ok'
return False

with a() as s:
print s


print s

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