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 setting forth the doctor's assessment of your functional limitations. The name derives from a section of 20 CFR 404.1527(d)(2), the regulation describing medical opinion evidence. See also 20 CFR 404.1513(b)(6).
The SSA must give controlling weight to a your treating doctor's opinion, if it is "well-supported by medically acceptable clinical and laboratory diagnostic techniques and is not inconsistent with the other substantial evidence in your case record." 20 CFR 404.1527(d)(2). This deference due to a medical source statement makes it very important for a disability claim.
Quite simply, a medical source statement may be the most powerful evidence that a lawyer can develop for a Social Security disability claim.
This issue is so important for every disability claim that I have added a medical source statement page to this site, and added a link to that page on the sidebar, under "Important Topics For Claimants." Available on that page are pdf copies of Social Security's medical source statement (physical) and medical source statement (mental). You can download the form you want, print it, and take it to your next medical appointment for your provider to complete.