Rezoning is a form of the term Zone, used as a verb. Rezoning is done in instanced areas by leaving the area then coming back to it. To rezone, any player goes through a portal, either to reset the monsters, bring all the party members to the same place and back to life, or to avoid death (changing zones to escape monsters).
For instance, when a runner has failed a run, he may tell a player to "rezone!", so everyone will return to life and the attempt can be made again.