This tag should be used for unconfirmed transactions, transactions that have not yet been included in a block.
An unconfirmed transaction is a valid transaction which has not been confirmed, i.e. it has not yet been included in a block. A transaction may be unconfirmed for many reasons: it was recently broadcast and a block has not yet been found, it's transaction fee is too low, it has many unconfirmed parent transactions, etc.
Unconfirmed transactions should generally not be accepted as a payment unless the receiver is highly confident that the transaction will confirm. Even so, high value payments should wait for a confirmation before considered final as double spending an unconfirmed transaction is fairly trivial, even without the Replace-By-Fee node policy employed today by many nodes.