what does "You must register or identify to your nickname to enter this channel." message means? i can enter this channel before, but i when tried to again, i got that message.

If you're on a bahamut-based (DALnet's ircd) network, that means that the channel is +R and you must /identify for your nick in order to get in.

i'm on aniverse network. how do i register or identify to my nick?

While you're on that network,

/join #Help

and ask in there. They will be able to tell you the specifics of how their network works.