The Celtic shield knot is used for warding and protection. Shield knots have appeared in cultures around the world and have taken a variety of different forms. They are almost universally square in shape, and the knotwork of the design ranges from simple to complex. In the Celtic version, a series of knots is formed. In other cultures, such as the early Mesopotamian era, the shield is simply a square with a loop at each of the four corners.
Fans of Celtic artwork occasionally get variations of this piece as tattoos or wear them as talismans of protection. In modern Celtic reconstructionist groups, the shield knot is sometimes invoked as a ward to keep negative energy away. In some traditions, the corners of the knot are meant to represent the four elements of earth, air, fire and water, although it's important to note that Celtic spirituality is usually based upon the three realms of earth, sea and sky.
If you're interested in following a Celtic Pagan path, there are a number of books that are useful for your reading list. Although there are no written records of the ancient Celtic people, there are a number of reliable books by scholars that are worth reading: Reading List for Celtic Pagans.
There are a great deal of symbols found in Celtic culture, including both knotwork and the Ogham alphabet symbols.
See our readers' Celtic knotwork tattoos: