A yoga wheel is pretty much exactly what it sounds like - a circular prop that you can lie on either forward or backward and roll underneath your body. Its primary purpose is to help you achieve advanced yoga postures and poses that require the support of a prop of this size. Different postures may require different size wheels, but generally one size can help accommodate a large range of different poses so you only really need one.
The wheel is designed to address particular physical problems and challenges centered around the spine. If you have back pain or stiffness, rolling around on a yoga wheel can help relieve these issues. Consider the circular shape of the prop, and how lying your back on it will emphasize the natural shape of your body when you relax. The front of your body will open up and allow you to fully stretch out. This is especially beneficial for anyone who has bad posture and finds themselves sitting or standing in a hunched over position where the chest is closed and the shoulders slumped in.
Staying in this position can cause back, neck, and shoulder pain, but a yoga wheel can help alleviate these problems. Simply use the wheel to lie backwards, and bring the chest and shoulders back to a fully prone position. These areas, along with the abdomen and hip flexors, become stretched out to reduce pain and stiffness.