Typing HELP on its own will provide you with a summary index, HELP INDEX will provide you with a full index for the entire Avalon helpsystem, and HELP CONTENTS - not for the faint-hearted - will provide you with a long list of titles covering each and every publicly available page of information documenting this wondrous and complex land of Avalon.
There are also other help-systems. Civic commands in CITYHELP and player-written citizen help in CH or CHELP. Guild helps in GUILDHELP or GH for short. Order help in ORDERHELP or OH for short. There is also skill help in AB <skill> <ability> and HELP TRUEINDEX for a short index of content not appropriate for the full HELP CONTENTS index (or the website).
The PRIESTHELP content is for priests and high priests only. Deities of course have their own divine help index of GODHELP files as do administrators. There is NOVICEHELP for younger players and player histories in HISTORY <persona>.
The best place to start for a beginner is HELP 1 - the first section which begins with HELP CONCEPT and will show its index if you type HELP 1, with all primary help sections listed when you type HELP INDEX and then HELP <section number> to see that section's list of actual files you can HELP <filename> to read over.