It is a Triquetra (pronounced "trai kwetrel"; Latin tri- "three" and quetrus "cornered") originally meant "triangle" and was used to refer to various three-cornered shapes. In Celtic Lore it represented the interconnected parts of our existence (Mind, Body, and Soul). Which is perfect for our fitness lives.
I feel that to experience life we must be centered if you will to those three elements of our life - Mind, Body and Soul. We must develop all three to remain healthy. Obviously as a Personal Train I advocate the physical fitness of the Body. But for our body to be truly "in shape" we must develop and keep the Mind sharp. Constantly learning, challenging our intellect will help protect us from degenerative mind and memory diseases. To make our Body and Mind complete we must nourish the Soul. Whether it be a formal religion, lifestyle belief or just living your life with a purpose it is important to have that "spiritual path" that will give your life meaning. Combine the three and you are completed.