In finance, a long position in a security, such as a stock or a bond, or equivalently to be long in a security, means the holder of the position owns the security and will profit if the price of the security goes up. Going long  is the more conventional practice of investing and is contrasted with going short. An options investor goes long on the underlying instrument by buying call options or writing put options on it.
In contrast, a short position in a futures contract or similar derivative means that the holder of the position will profit if the price of the futures contract or derivative goes down.
See also 
- Harrington, Shannon D. and Tim Catts, Sep 13, 2010, "Bond Buyers Getting Burned by Going Long as Yields Climb: Credit Markets", Bloomberg News