A money market is more or less a mutual fund that attempts to keep its share price at $1. Professional money managers will take your cash and invest it in government t-bills (aka "treasuries"), savings bonds, certificates of deposit, and other safe and conservative short term commercial paper. They then turn around and pay you, the owner of the money market, your portion of the interest earned on those investments.
Most banks offer money market accounts to their customers, although the amount of interest paid will vary by account size. Generally, the highest interest rates are paid to those who invest $100,000 or more.
Money market accounts are frequently used to park cash between investments.
