max_fsm_pages

I just did a vacuum analyze and I got a message I've never seen before:

conwatchlive=# vacuum analyze;
NOTICE:  number of page slots needed (27056) exceeds max_fsm_pages (20000)
HINT:  Consider increasing the configuration parameter "max_fsm_pages"
to a value over 27056.
VACUUM
conwatchlive=#

What does this mean? I assume it has nothing to do with the Flying
Spaghetti Monster.

More generally, I am a novice at administering a PostgreSQL database, is
there a list of tips somewhere that I can read to improve performance?