Federal funds are the reserve balances (also called federal reserve accounts) that private banks keep at their local Federal Reserve Bank 1, 2. These balances are the namesake reserves of the Federal Reserve System. The purpose of keeping funds at a Federal Reserve Bank is to have a mechanism for private banks to lend funds to one another. This market for funds plays an important role in the Federal Reserve System as it is what inspired the name of the system and it is what is used as the basis for monetary policy. Monetary policy works partly by influencing how much interest the private banks charge each other for the lending of these funds.
Federal reserve accounts contain federal reserve credit, which can be converted into federal reserve notes. Private banks maintain their bank reserves in federal reserve accounts.