"Consider compacting this relation..." ???

I just ran VACUUM ANALYZE and got this warning I've never seen before:

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

What does the hint mean by "compacting"? What exactly must I do to compact
this table?

Also, at the end of the VACUUM ANALYZE run I get the NOTICE:

NOTICE: number of page slots needed (294528) exceeds max_fsm_pages (153600)
HINT: Consider increasing the configuration parameter "max_fsm_pages" to a
value over 294528.

What surprises me here is that even though I've never seen this notice
before (and I've been running VACUUM ANALYZE a lot lately!), the number of
page slots needed is almost twice as much as the value of max_fsm_pages.
What could I have done since the last time I ran VACUUM ANALYZE (not more
than 24 hours ago) to make this "page slot" requirement to shoot through the
roof like this?

Thanks in advance!

kynn