The question arises from these powerful lines as to who the "idiot" telling the "tale" is. For Macbeth, now fallen to his moral depths, "it" signifies "nothing" and is life mentioned a few lines earlier: "Life's but a walking shadow, a poor player/That struts and frets his hour upon the stage...."
... it is a tale
Told by an idiot, full of sound and fury,
Signifying nothing. (V.v)
The "it" that is a "tale" and also a "walking shadow"is "Life."
I suggest that the "tale" told by "an idiot," a tale consists of those dark forces that tempted him to murder to begin with. The witches told a tale--or suggested to him the possibilities of power and revealed to him his own ambition--that in the long run lacked meaning. Earlier the play associates "equivocal" with the witches; in these lines they no longer equivocate, they simply lack meaning--they are a lie.
"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." The "it" is life itself.
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).