The Celtic shield is often used to ward off evil spirits, and also for protection.
Many people across the world have received tattoos or talismans of this object so that they can do this. Usually in this symbol it is always square, possibly to represent the 4 elements, earth, air, fire, and water. Sometimes in Celtic artwork you will see this symbol drawn in more detail with many more knots or more complex. This is easily seen throughout ancient Mesopotamia.