The supplementary motor area (SMA) is a part of the primate cerebral cortex that contributes to the control of movement. It is located on the midline surface of the hemisphere just in front of (anterior to) the primary motor cortex leg representation. In monkeys the SMA contains a rough map of the body. In humans the body map is not apparent. Neurons in the SMA project directly to the spinal cord and may play a role in the direct control of movement. Possible functions attributed to the SMA include the postural stabilization of the body, the coordination of both sides of the body such as during bimanual action, the control of movements that are internally generated rather than triggered by sensory events, and the control of sequences of movements. All of these proposed functions remain hypotheses. The precise role or roles of the SMA is not yet known.

Source : Wikipedia

There are currently four theories regarding its function. That it has a role in postural stability - that it co-ordinates bi-manual activities - that it controls the initiation of thoughtful action (as opposed to stimulus driven action) - and/or that it controls timing functions of deliberate actions.

"The data, however, tend not to support an exclusive role of SMA in any one of these functions."