The Triquetra or the Trinity Knot is comprised one continuous line interweaving around itself, meaning no beginning or end, or eternal spiritual life.
This symbol was originally Celtic, not Norse, but with increased contact and assimilation between the Vikings and the peoples of Ireland and Scotland, the Triquetra and other Celtic symbols/motifs became culturally syncretized.
Originally, the Triquetra was associated with the Celtic Mother Goddess and depicted her triune nature. The triple identity was an essential feature in many aspects of druidic belief and practice.
Later, Irish and Scottish monks adopted the Triquetra as a symbol of the Christian Trinity. People today wear the Triquetra for any of these reasons and to be reminded of the continuity and multi-faceted nature of higher truths.