It's covered in help, so if you forget you can reference it there :-)
Under help > Games > Threads and Messages > Thread Groups.
Public: Only the GM can post to Public threads, but all users of RPoL can read them.
Group 0: Only GMs and characters in group 0 can read and post to Group 0, but all visitors can read it.
That means people need to be logged into RPoL to read Public threads (RPoL users), but anyone can read Group 0 threads (visitors).