This is the current depth of the unreliable transmit queue on the interface -- EIGRP packets such as Hellos that do not require acknowledgement. Because Hello packets are what keep an adjacency alive, a backed-up unreliable queue can delay Hellos enough to trigger a hold-timer expiration and adjacency loss even though the link itself is fine. If this queue sits at 15 while Hellos are supposed to fire every 5 seconds, that backlog alone could push a Hello out past the hold-time window on a slow-CPU router.