Mailman has a large number of site-wide configuration options which you should now review and change according to your needs. Some of the options control how Mailman interacts with your environment, and other options select defaults for newly created lists5. There are system tuning parameters and integration options.
The full set of site-wide defaults lives in the $prefix/Mailman/Defaults.py file, however you should never modify this file! Instead, change the mm_cfg.py file in that same directory. You only need to add values to mm_cfg.py that are different than the defaults in Defaults.py, and future Mailman upgrades are guaranteed never to touch your mm_cfg.py file.
The Defaults.py file is documented extensively, so the options are not described here. The Defaults.py and mm_cfg.py are both Python files so valid Python syntax must be maintained or your Mailman installation will break.
You should make any changes to mm_cfg.py using the account you installed Mailman under in the 3 section.