The Jewish year is lunar, of 354 days, and thus a thirteenth month must be intercalated every few years to even it out with the solar year, of 365 days.  A month is added on after the twelfth  month, Adar, and for this reason is called Adar Bet, or Adar B.  Passover falls in the first month of the year, in Nissan.  Thus, following a leap year, Passover occurs relatively late.