Settle an account, close a matter, quit, as in I'm simply going to cash in and leave, or The countries of the former Soviet Union have cashed in. [Late 1800s]
Profit handsomely, as in When the stock price went up, we really cashed in. This phrase often is extended to
cash in on, meaning to take advantage of. [Early 1900s]
Also,
cash in one's chips. Die, as in If this new treatment fails, Bob may be cashing in his chips before long. This usage was a transfer from quitting a poker game. [Slang; late 1800s]