The Fed maintains the federal funds rate - the interest rate at which US banks make overnight loans to each other - at (or close to) the Fed's chose target by injecting (the rate falls) and extracting (the rate rises) liquidity from the banking system. The Fed does this through repurchase agreements (repos) or by manipulating the stock of Treasuries on its balance sheet.
However, when the Fed wants to keep the federal funds rate unchanged but add liquidity by other means (perhaps by auctioning off funds), the Fed sterilizes the effects of the new liquidity on the monetary base by performing an offsetting open market operation - an overnight repo or selling Treasuries outright.
Leaving money in the banking system as excess reserves increases the supply of federal funds & reduces the federal funds rates (below the Fed's target). The increased money supply is inflationary-too much money chasing the same number of goods & services.
