what does __file__ mean in getting abspath

[Malt]

I'm exploring Django framework and in manage.py, I could see statement
BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

I can understand till os.path.dirname and os.path.abspath, but what does __file__ mean?. Are we defining it anywhere?
I'm newbie to Python development, please bare my queries