Your Residual Functional Capacity (RFC) is the cornerstone of your Social Security disability claim. Unless you meet a listed impairment, the administration's assessment of your RFC will decide the outcome of your disability claim.
The RFC is Social Security's assessment of your abilities to do sustained physical and mental activities on a regular and continuing basis in a work setting. The RFC considers only those funtional limitations resulting from medically determinable impairments.
The RFC is used to determine whether or not you can return to your past relevant work (step 4 of the sequential evaluation process) or do other work (step 5 of the sequential evaluation process).
For more about your RFC, read Social Security Ruling 96-8p.