Can someone explain __all__ in Python?

__all__ = ['bar', 'baz']

waz = 5
bar = 10
def baz(): return 'baz'

from foo import *

print bar
print baz

# The following will trigger an exception, as "waz" is not exported by the module
print waz

a = "A"
b = "B"
c = "C"

__all__ = ['a', 'b']

a = "A"
b = "B"
c = "C"

__all__

__all__