[Tutor] Beginner Q: What does the double underscore mean __ ?

[Aaron Pilgrim]

Hi,
I was trying to teach myself some python from a book and I have seen
double underscores used.
What does the __ mean and how do I know when to use it?

for example:

__name__==’__main__’

(source)
Pilgrim, Mark (2009-10-23). Dive Into Python 3 (Books for
Professionals by Professionals) (Kindle Locations 480-481). Apress.
Kindle Edition.