The phrase originates from casinos where players exchange chips for money when they're done. He played blackjack all night and finally went to cash in his chips.
Pro Tip 2/3
Literal Use
Used literally when talking about actual casino chips being turned into cash. Sarah just left the casino to cash in her chips.
Pro Tip 3/3
Success Cue
Often implies leaving after achieving success, typically in gambling. After hitting the jackpot, he decided to cash in his chips.