Warning about max_fsm_pages: what's that?

Hi all.
During a routine "VACUUM ANALYZE" I've got this message in the logs:

WARNING:  relation "public.t_dati" contains more than "max_fsm_pages" pages
with useful free space
HINT:  Consider compacting this relation or increasing the configuration
parameter "max_fsm_pages".


The table in question contains some 14M+ rows.

Would it be possible to get some reference or suggestion in order to
understand better the provided hint?
This is the very first time I see this message.

Many thanks.

--
Reg me Please