What is " a tale told by an idiot, full of sound and fury, signifying nothing"?

"Life's but a walking shadow, a poor player, That struts and frets his hour upon the stage, And then is heard no more.  It is a tale told by an idiot, full of sound and fury, signifying nothing."

Macbeth has just heard the news of his wife's death (suicide), and is voicing the pointlessness of existence (his and his wife's, at this point).