Grayskull was the name of a group of First Ones freedom fighters which included Mara and Serenia. Mara used the phrase "For the honor of Grayskull" to transform herself into She-Ra, and so did the next recipient of the Sword of Protection, Adora.
Very little is known about this faction, as they were defeated by The Horde a thousand years prior to the coming of the new She-Ra. Bow and Glimmer eventually learned the meaning of the name Grayskull from the Hologram in the ruins of the Crystal Castle, but Adora was elsewhere at that moment.