A section of the Stora Hammars I stone in Gotland representing a so-called valknut (the three triangles), alongside a figure with a spear, which might be interpreted as Odin, accompanied by what looks like a raven and a hanging man near a burial mound.
The term valknut is a modern one, built up to mean the knot of the slain, the dead men on the battlefield. The places where this symbol appears do seem to be set in a context related to death, yet its significance is uncertain. The name of the object in the Viking Age is unknown. It might be what Icelandic scholar Snorri Sturluson describes as the heart of Hrungnir in Skáldskaparmál (The Language of Poetry) in his Edda from the 1200s: Hrungnir had a famous heart made of hard stone, with three corners, that later becomes a design by the name of Hrungnir's heart.
We find the symbol not only on the Stora Hammars and Tängelgårda rune stones from Gotland but also on other objects such as a ring (Nene River Ring) or a bedpost from the Oseberg ship. We can only speculate that it was linked somehow to Odin's spiritual and magical abilities, his links to the afterlife, or his ability to bind, both to inflict fear and to induce a state of frenzy. They might have been a broader category in the Viking/Germanic world, sets of three objects like this we also find on the Snoldelev runestone from the 9th century in Denmark, a regular memorial stone with an inscription in the younger futhark alphabet. Again, we can speculate around the three horns as potentially magically charged, or simply an attractive design. There is no way to tell for sure.