A wobble board is a piece of training equipment used to develop physical balance. It is often used for rehabilitation purposes, although it can be very useful to improve balance and reflexes.
See also Wikipedia's Balance board article.
The wobble board is a wooden disk. The top side, on which the individual stands, is flat and usually features a non-slippery cover. The bottom side, which goes on the ground, has a hemisphere in the center. This allows the board 360 degrees of movement, usually with 10 to 20 degrees of axial tilt.
The most common exercise done is to stand on the wobble board and move from front to back without the sides touching the ground, or move from side to side without the front and back touching the ground. It is however possible to use the wobble board in a wide variety of exercises, such as balanced push-ups and sit-ups, or to improve balance on one leg.