In this scene, Mortimer makes his first and only appearance in the play. He is the King's cousin, and, at the opening of the scene, is setting the stage for the audience as to his condition -- He is dying in jail.
He refers to his jailer as the "keeper of [his] weak, decaying age," and asks that they let him rest. He compares the state of his "limbs" (arms and legs) to a man who has just been tortured and stretched on a "rack", this the result of having been in prison for so long. He compares his hair to a herald announcing his death (because of their color) and also refers to them as being "Nestor-like." Nestor was a Greek king who lived three lifetimes, agian emphasizing his own advanced age.
His "eyes," he compares to "lamps" that have no more "oil" (ie, no more light in them); his "shoulders" have been bent over with "grief;" his arms are as the "wither'd vine/That droop sapless branches to the ground;" and his "feet" are "numb" and "unable to support this lump of clay [his body]."
All of these metaphors are meant to give the audience the image of how old, tired and at the very last breaths of his life on earth Mortimer is. This teetering on the precipice of death is a great suspense builder as he prepares to address the King. The question is planted in the audience's mind -- "Will Mortimer live through the telling of his story???"