Basically it contains all the attributes which describe the object in question. It can be used to alter or read the attributes.
Quoting from the documentation for __dict__
A dictionary or other mapping object used to store an object's (writable) attributes.
Remember, everything is an object in Python. When I say everything, I mean everything like functions, classes, objects etc (Ya you read it right, classes. Classes are also objects). For example:
def func():
pass
func.temp = 1
print(func.__dict__)
class TempClass:
a = 1
def temp_function(self):
pass
print(TempClass.__dict__)
will output
{'temp': 1}
{'__module__': '__main__',
'a': 1,
'temp_function': <function TempClass.temp_function at 0x10a3a2950>,
'__dict__': <attribute '__dict__' of 'TempClass' objects>,
'__weakref__': <attribute '__weakref__' of 'TempClass' objects>,
'__doc__': None}