A repurchase agreement, or repo for short, is a type of short-term loan much used in the money markets, whereby the seller of a security agrees to buy it back at a specified price and time. The seller pays an interest rate, called the repo rate, when buying back the securities.
Central banks often use repos to boost money supply, buying Treasury bills or other government paper from commercial banks so the banks can boost their reserves, and selling the paper back at a later date. When the central bank wants to tighten money supply, it sells the paper first, and buys it back later - this is called a reverse repo, an agreement to lend securities rather than funds.
Repo is also short for repossession.
Interactive graphic: how the repo market works