Wikipedia
Barrow-wights are wraith-like creatures in J. R. R. Tolkien's world of Middle-earth, based on Old Norse beliefs such as Draugr or vǣttr (wights). Barrow refers to the burial mounds they inhabited and wight is a Middle English word for "living being" or "creature", especially "human being". It does not necessarily mean "spirit" or "ghost"; it is cognate to modern German "Wicht", meaning small mythical creatures (also "Wichtelmännchen"). Tolkien borrowed this concept from Norse mythology, see e.g. Waking of Angantyr and Hrómundar saga Gripssonar. The name Barrow-wight itself was first recorded in 1869 in the Eiríkr Magnússon and William Morris translation of Grettis saga, which features a fight with such a creature. In Norway, farmers of the 19th century were still concerned about "vetter" (wights) around old viking barrows, when these were first excavated.
Evil spirits (perverted Maiar or possibly spirits of Orcs, fallen Avari, or evil Men) were sent to the Barrow-downs by the Witch-king of Angmar in order to prevent the restoration of the destroyed Dúnedain kingdom of Cardolan.
They animated the dead bones of the Dúnedain buried there, as well as older bones of Edain from the First Age which still were buried there.
After leaving Tom Bombadil, Frodo Baggins and company were trapped in the Barrow-downs, and nearly slain by a barrow-wight. It was mentioned in The Lord of the Rings Appendix A that Frodo was trapped in the cairn of the last prince of Cardolan; Merry's exclamation on waking from his trance suggests this. Frodo sliced off the wight's hand; then, when the wight extinguished the dim light in the cavern where the company was imprisoned, Frodo called upon Tom Bombadil, who expelled the wight from the barrow.