The Valknot or "knot of the slain" is an old Norse symbol that often represented the afterlife in carvings and designs. It is often considered a symbol of the Norse god Odin. Some white supremacists, particularly racist Odinists, have appropriated the Valknot to use as a racist symbol. Often they use it as a sign that they are willing to give their life to Odin, generally in battle.
Nonracist pagans may also use this symbol, so one should carefully examine it in context rather than assume that a particular use of the symbol is racist.