This is a plaque with a valknot design burned into it.
This symbol found on old Norse stonecarvings is called "Hrungnir's heart," after the legendary giant of the Eddas. It is best known as the Valknut, or "knot of the slain," and it has been found on stone carvings with funerary motifs, where it signified the afterlife.
The valknut can be drawn unicursally (in one stroke), making it a popular talisman of protection against spirits.
The Valknut's three interlocking shapes and nine points suggest rebirth, pregnancy, and cycles of reincarnation. The nine points are also suggestive of the Nine Worlds (and the nine fates) of Norse mythology. Their interwoven shape suggests the belief of the interelatedness of the three realms of earth, hel, and the heavens, and the nine domains they encompass.
The Valknut is also an important symbol to many follwers of the Asatru faith, who often wear it as a symbol of the faith.