What is XMPP?
For friends who do not know about 
XMPP, it is a "choice" other than "choices" we usually chose to communicate online in the internet.
So, loosely saying there are WhatsApp, Telegram, Signal, Matrix, Jami and there is XMPP. It can be your next choice of communication.
Find more explanation at the official source https://xmpp.org.
You might find this post from the XMPP poll above or #XMPP hashtag at Mastodon.
@ademalsasa Sure, you can reach me at me@matthewwild.co.uk by XMPP or email.
You should probably know in advance that I don't use OMEMO on my public XMPP account 🙂
@mattj very nice of you. Sent instantly, Matt. Thanks a ton.
Mental Outlaw's YouTube video "How to Setup Your Own XMPP Server" has a good introduction to what it is.
@ademalsasa Worth pointing out that the primary audience for xmpp.org is a technical one - it contains all the specifications for software developers to build their libraries and apps on top of #XMPP, and have it interoperable with the rest of the XMPP ecosystem.
It does have a few pages to help guide non-technical users, but a more user-focused intro to the network can be found at https://joinjabber.org/