What is XMPP/Jabber?
    "Extensible Messaging and Presence Protocol is an open communication protocol designed for instant messaging, presence information, and contact list maintenance. Based on XML, it enables the near-real-time exchange of structured data between two or more network entities."

How to sign up with XMPP/Jabber?
https://danwin1210.de/mail/register.php
Dan is my personal favorite, however you can usually register an account with whichever you decide to primary in

Clients?
For Windows:
https://gajim.org/

For Android:
https://conversations.im/
https://f-droid.org/en/packages/de.monocles.chat/ (Requires F-droid)

For Linux:
https://xmpp.org/software/profanity/
https://astrachat.com/

For Browser:
https://conversejs.org/

For Apple:
https://poez.io/en/

Not listed? View: https://xmpp.org/software/

But are we OMEMO yet?
https://omemo.top/