What is avahi in Linux?

avahi is a Linux implementation of a protocol also known as "Rendezvous" or "Bonjour"). Its goal is to let devices, connected to the local network, broadcast their IP-address together with their function. Hence the printer can from time to time broadcasts : My IP is 192.168.23.45 and I can print any postscript document with ipp prottocol; a NAS can say: My IP is 192.168.23.88 and I can stream music, save your backups, and act as a fileserver.

If it is not what you want to hear on your network you can stop / disable the avahi daemon with the standard systemctl command, but if you run a cups-broadcast daemon, it will start the avahi itself.

Linux uses fictive users usually for security reasons, not to give the attacker any chance to hack a process owned by root. So you can see a postfix or mail, and postgres or mysql users. The daemon, owned by such unprivileged user, gives less chance for the attacker to get the superuser rights.