People always ask me what particular knots symbolize, but it's really not an easy question to answer.  Though many folks will tell you that a triangular (Triquetra) knot represents the Holy Trinity (Father, Son, and Holy Spirit), other folks will tell you it symbolizes the life journey (childhood, adulthood, old age) or the three feminine powers (Maiden, Mother, Crone), and still others will tell you it represents the mind, body, and spirit.  The truth is, it's all of these things, to different people, and quite possibly none of these things to the originator of the knot! 

Decorative knotwork and interlace have appeared in Roman, Coptic, Byzantine, Islamic, Ethiopian, and European art and architecture as far back as the 3rd century AD.  Knotwork illumination possibly came to the Celtic peoples as early as 450 AD, in early Christian manuscripts.  However, the earliest surviving example of true Celtic knot designs (a fragment of a Gospel Book) was created in the 7th century AD.  Many historians believe this is when the practice of decorative knotwork reached Ireland, subsequently spreading to Scotland (Pictland and Dalriada), Wales, and Northumbria.  Viking raiders modified the Celtic knotwork designs into the zoomorphic style of interlaced animal and human forms.

Many religions and groups over the centuries have adopted various knots for their own purposes, assigning them meanings that fit within their belief system.  So, "what does this know symbolize" is really a comlicated question to answer!  I tell my customers to decide what the knot means to them.  Everyone sees something different in the neverending interlace of a Celtic knot, and no interpretation is wrong!  Choose the symbol that symbolizes you.