The holder of the position has the expectation that the financial instrument will increase in value.
The term "long position" is often used in context of buying options contracts.
[2] When an investor holds a long position in a stock they are buying a share of ownership in a company.
Going long in a future means the holder of the position is obliged to buy the underlying instrument at the contract price at expiry.
This is different from going long by buying the underlying or trading in futures, because a long position in an option does not necessarily mean that the holder will profit if the price of the underlying instrument goes up.