RMP is an abbreviation for the Usenet newsgroup rec.music.phish.
Newsgroups were threaded discussion lists that predated the world wide web. At one time you needed a specialized software client to read newsgroups; later on they were taken over by googlegroups.
Rec.music.phish was one of the first Usenet newsgroups, but at the time it (as well as this website which came along in 1994 to post setlists and the FAQ) were both known as "phishnet", that is, prior to this site "phish(dot)net". It's confusing, but pre-world wide web in 1994 or so, there were a bunch of other email lists, FTP sites, usenet newsgroups and such that were operated by the fans under the name "phishnet". There were also news announcements of setlists and such which exist to this day, as well as email digests of rec.music.phish during the mid-late 90s by volunteer editors Rosemary McIntosh Speno and Benjy Eisen.
As shown by the above discussion, much of rec.music.phish is searchable on googlegroups if you want to check it out.
Phish.net is a non-commercial project run by Phish fans and for Phish fans under the auspices of the all-volunteer, non-profit Mockingbird Foundation.
This project serves to compile, preserve, and protect encyclopedic information about Phish and their music.
The Mockingbird Foundation is a non-profit organization founded by Phish fans in 1996 to generate charitable proceeds from the Phish community.
And since we're entirely volunteer – with no office, salaries, or paid staff – administrative costs are less than 2% of revenues! So far, we've distributed over $2 million to support music education for children – hundreds of grants in all 50 states, with more on the way.