from wikipedia.

The pension amount is determined by a formula that takes into account the years served and the average pay for the top three years in terms of payment. In 2002, the average pension payment ranged from $41,000 to $55,000. For example, a member of Congress who worked for 22 years and had a top three-year average salary of $153,900 would be eligible for a pension payment of $84,645 per year.[3]

This is to supplement social security.