"The circle, being primary, influences how we as Aboriginal peoples view the world. In the process of how life evolves, how the natural world grows and works together, how all things are connected, and how all things move toward their destiny. Aboriginal peoples see and respond to the world in a circular fashion and are influenced by the examples of the circles of creation in our environment". 
There isn’t a simple answer to the question as medicine wheels (sometimes called hoops) come in more than one form, and their significance and use is culture-specific. There is, however, one fundamental similarity besides the shape - medicine wheels represent the alignment and continuous interaction of the physical, emotional, mental, and spiritual realities. The circle shape represents the interconnectivity of all aspects of one’s being, including the connection with the natural world. Medicine wheels are frequently believed to be the circle of awareness of the individual self; the circle of knowledge that provides the power we each have over our own lives.