It means exactly what it seems. A simple and
common proverbial phrase, arguing what goes up must come down.
The example below is an except from 'Human
Traffic' (1998), an independent
British film said by character Jib.
What goes up must come down, and down, and down. Everyone looks
ill at the end of the
night. All have lost the power of speech, desperately avoiding eye contact. Your new
soulmate, that you have been talking cod shit to for the past five hours about the story of creation or the fourth Star Wars film, is now a complete stranger.