Complex sentence: 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.
Split into simpler sentences: In finance, a long position in a finance instrument, means the holder of the position owns a positive amount of the instrument. In terms of 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.