There are few symbols with Celtic recognition as the Celtic Knot, which is why we have a plethora of Celtic Knot jewelry. As with most ancient symbols and cultures, we are not exactly sure what the Celtic symbol meaning is, but we can use context in archaeological finds, Celtic art, and Celtic jewelry to surmise its true meaning. It is generally accepted the Celtic Knot refers to the never-ending cycles in life. One of the most popular versions is the Triquestra, symbolizing:
Christians came over and adopted it as the Trinity Knot, the 3 parts of the Holy Spirit. You can of course find Trinity Knot Jewelry as well.