Well that’s kind of simple but here are some more details about it so you can understand it better:

Database tests:  Tests executed on your already existing users.
That could be WordPress users, AcyMailing users or any other users already present in your database.

New subscription tests: This refers to all the new users trying to subscribe to your website.
That includes new WordPress uers, new AcyMailing subscribers etc.

So that when a user tries to register to your website then you do some tests to make sure it is not a fake one.