Does underscore has any special built-in meaningin Python ?

[dandi kain]

Hello everybody,
I have just started learning Python.I heard its simple so I pick a
presentation [1] and tried to work on it.But when it comes to
underscores leading and trailing an object I dont understand any.I
look through the python manual also but that was not helping .I
searched some forums and I still dont have a clear picture.

What is the functionality of __ or _ , leading or trailing an object ,
class ot function ? Is it just a naming convention to note special
functions and objects , or it really mean someting to Python ?

Thanks ahead ,

[1] http://www.aleax.it/goo_py4prog.pdf