He used to be a member of the Galactic Empire until he defected to the Rebel Alliance. It is he who came up with the plan to steal the plans to the Death Star.
After the Death Star plans were successfully stolen and made their way to a Rebel base, Jan was part of the Rebels who designed an attack strategy based on those plans, which lead to the Battle of Yavin. This lead to the destruction of the Death Star. Jan was part of the ensuing celebration.
After that, Jan continued to lead the Rebel Alliance into battle until the death of Palpatine at the hands of the redeemed Anakin Skywalker, which lead to the fall of the Empire.
Dodonna was The First Character in Star Wars to say the iconic line May The Force Be With You