Being disabled means you can't work. Step 1 of Social Security's 5-step sequential evaluation process is a determination of whether or not you are working. To make that determination, Social Security looks at whether you are engaged in substantial gainful activity (SGA). If you are an employee and a making more than this amount each month, Social Security concludes that you are engaged in SGA. As a result, you are not disabled under Social Security's rules.
This is a very important topic. As a claimant, if you have been working at the SGA level since the onset of your disability, you can lose you disability case before the judge ever considers the severity of your medical impairments.