A medical source statement is an opinion from your doctor regarding the nature and severity of your limitations due to your medically-determinable impairments. A medical source statement is often simply a form that is completed and signed by your doctor. The name derives from a section of the regulation describing medical opinion evidence.

The SSA must give controlling weight to a your treating doctor's opinion, if it is well supported by the evidence in the case record. This deference given to the medical source statement makes it very important for a claim.

Quite simply, a medical source statement is the most powerful evidence that a lawyer can develop for a Social Security disability claim. 

{update: here is the medical source statement (physical) form that I use in my practice}