The Ghost Fleet Offensive was an initiative by the Colonial Fleet to put Cylon forces on the defensive by using ships thought to have been destroyed but were actually hidden away.
The plan involved feeding misinformation about the Ghost Fleet's position to the Cylons, who then scrambled most of the ships defending their bases. However, once the Cylon fleet arrived at the Ghost Fleet's position, they were long gone. With the Cylon bases now undefended, the Colonials struck, inflicting major damage on "every toaster base in five sectors," in the words of Commander Silas Nash. In addition to weakening the strategic position of the Cylons, the offensive provided a much needed morale boost for the beleaguered Colonials (Blood and Chrome).