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.