Open market operations occur when the Fed buys/sells securities which primarily tend to US Treasuries. The Fed uses Treasuries because they are the most commonly held asset in the US and thus can handle the large operation of the Fed.

When the Fed buys US Treasuries is increases the money supply since the Fed exchanges newly created money for assets. Likewise, when the Fed sells Treasuries, it decreases the money supply. This is because the Fed is exchanging assets for money and will keep the money out of circulation.