Your benefit as a wage earner is based on your "average indexed monthly earnings" (AIME). This is a history of your lifetime earnings as it is maintained in the records of the Social Security Administration (SSA).
If you were born after 1928, your AIME is based on your highest paid 35 years of work. This includes work for yourself, as well as work for employers who are covered by social security, and also includes time spent in the military. The social security system assumes a 40-year career and uses the 35 years of the highest earnings to compute your AIME. If you did not work the minimum number of years, you will be adding zeros into the average, thereby greatly reducing your AIME and subsequent benefit amount. If you work more than forty years, the SSA will still use your 35 highest-paid years.