You may have a Guaranteed Minimum Pension scheme if you were a member of a  defined benefit  (also known as a final salary) public sector pension scheme between 1978 and 1997. Your GMP calculation was set to provide members with a pension that was at least as much as that which they would have earned under the SERPS payable from 60 females or 65 males (in line with the state Pension ages during that period).
More info - https://www.gov.uk/government/publications/guaranteed-minimum-pension-checker-service-availability-and-issues