What is the meaning of +r?

Usman

+r is a channel mode which restricts the channel to "Registered Users Only", meaning that only users who are logged in to X can join the channel.

If you're an operator, and you want a non-registered user to join your channel, you can "override" this mode with the /invite command: /invite nickname #channel
Also, if you're an operator in the channel, you can unset this mode with: /mode #channel –r