The term "money market" refers to a financial market in which participants can borrow and lend money for short periods, usually ranging from one day to just under a year. The money market is used by banks, corporations, and governments to manage their cash flow and make investments.
When someone says that they are "in the money market," it means that they are actively participating in this financial market as either a borrower or lender of funds. So, when you hear the phrase "one's money market" it refers to an individual's involvement and participation in the money market.