setup.py is a python file, Which has all the detail about the module/package you are about to install has been packaged and distributed with Distutils.
The Distutils is the standard for distributing Python Modules.
So, setup.py file allows you to easily install Python packages. So by writing the following command, you can easily install setup.py.
$ python setup.py install
To know more about this you can have a look at the following video tutorial:-