A money market fund (MMF) is a mutual fund that buys very short-term, high-quality debt, Treasury bills, repo, bank CDs and commercial paper, and passes the yield to investors. Because the holdings are so short-dated, many MMFs quote a stable $1.00 NAV and feel like cash. They are not cash. There is no bank, no deposit, and crucially no FSCS / FDIC insurance: you own fund shares, and shares can fall in value.