Imperial Death warrants were Imperially sanctioned death sentences that were acted upon by bounty hunters, Imperial Assassins, and other Imperial agents.
After having been rescued by Galen Marek, Senator Garm Bel Iblis cited Imperial death warrants on himself and his family as one of the reasons that convinced him to join the Rebellion.
In 0 ABY, Stormtroopers on The Wheel tried to enact an Imperial death warrant on Chewbacca. In 3 ABY, Darth Vader was unable to revoke the warrant issued against Payback, but was able to issue a reprieve.
After helping to rescue Han Solo from Jabba the Hutt, R2-D2 sliced into Jabba's palace computer network to upload false evidence of Imperial death warrants on the late crime lord's chief surviving lieutenants. In response they began to flee the planet en masse, creating a diversion that allowed Artoo, Solo, and the other Rebels to sneak past an Imperial blockade.