phpMyAdmin is a tool that lets you peek and poke directly into your MySQL database.
It is a PHP application just like b2evolution, except it has nothing to do with b2evolution.
Nevertheless, you can use phpMyAdmin to connect to your b2evolution database and see how b2evolution stores its data there. You may also use it to fix problem that occurred in some rare occasions.
Normal users should never need to use phpMyAdmin and should never need to look directly into their database. Do this only at your own risk. Changing the DB may break your b2evolution website.