Jitsi Meet: What is XMPP used for?

I installed jitsi meet on my Debian server, and it is working fine.

What I do not understand is where XMPP is coming into the game. What is it used for? An example would be nice.

The manual installation docs say that xmpp is accessed through the web server, and that the xmpp servertalks with jicofo.

Also, does that mean that I can use a normal XMPP chat client with jitsi meet?
Can I use my normal xmpp accounts for authenticated video conferences in meet?

The prosody xmpp server is central in the jitsi-meet system architecture.
jicofo is configured to be an administrator on the prosody xmpp server.
The video bridge and other jitsi modules such as jicofo are users on the prosody xmpp server.
The system can be expanded to cover various usecases.

                           end user
                webb-browser javascript and webrtc
                     or dedicated app on mobile
                   ^                           ^
                   |                           |
                   |                           |
                   |       public ip           |
               +---+---------------------------+---+
               |      router with port forward     |
               +---+---------------------------+---+
                   |                           |
                   |       local ip            |
               +---+---------------------------+---+
                   |  debian server            |
                   |                           |
                   |                           |
                   v                           |
                  443                          |
               +-------+                       |
               |       |                       |
               | Nginx |                       |
               |       |                       |
               +--+-+--+                       |
                  | |                          |
+------------+    | |    +--------------+      |
|            |    | |    |              |      |
| jitsi-meet +<---+ +--->+ prosody/xmpp |      |
|            |files 5280 |              |      |
+------------+           +--------------+      v
                     5222,5347^    ^5347   4443,10000
                +--------+    |    |    +-------------+
                |        |    |    |    |             |
                | jicofo +----^    ^----+ videobridge |
                |        |              |             |
                +--------+              +-------------+