In the lines: "life's but a walking shadow, a poor player that struts and frets his hour upon the stage", Macbeth is expressing that life lacks substance and he is also comparing it to the life of an actor who is performing and all of a sudden he is no longer doing so.
In the lines: "it is a tale/told by an idiot, full of sound and fury signifying nothing", Macbeth is comparing life to a story told by someone who lacks intelligence; therefore, it is sort of meaningless.
