What kind of product is an MMF (money market fund)?

An MMF is a fund that invests in short-maturity, low-credit-risk money market instruments such as government bonds and Monetary Stabilization Bonds, high-grade commercial paper (CP) and certificates of deposit (CDs).

Because same-day deposits and same-day redemptions are possible, it is widely used to park spare cash temporarily. However, an MMF is still a fund, so principal is not guaranteed.