But what is a jabber?

Extensible Messaging and Presence Protocol (XMPP, originally named Jabber) is an open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance.

What's a good client for it?

Generally speaking, Gajim is the most modern one, Dyno is the most stable one, Psi supports the most features while not using GTK4 and not crashing after pressing a single button, and Conversations (or forks like Blabber.im) are the most stable and feature rich ones for Android.

How do i log in?

First, you need to register your account. Then you can log in with your credentials, such as username@adastra7.net (this is what is called a JID, not an email address) and your password of choice. Your client shuold automatically connect to adastra7.net at port 5222.