In this particularly poignant speech, Macbeth is referring to his way of life. He comparies his life to a withered yellow leaf. And indeed, things have not worked out as he planned. He is fully aware that Lady Macbeth is a victim of a "mind diseased" and that all their crimes have brought sorrow--
Which weighs upon the heart.
The threat of Malcolm and Macduff joining forces and marching on Dunsinane is fast becoming a reality, and Macbeth sees no true means of escape. In this desperate hour, Macbeth evaluates his life and knows that he has achieved nothing:
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have . .,
Approaching what seems to be the end of his life--"the yellow sear," he feels as if he has lived long enough. His life has withered instead of flourished, and like a dead leaf he can only wait for the final winds--the English forces-- to blow it from the tree into oblivion.
This line is found in Act V, Scene 3.
I would say that this imagery is suitable to the outcome of the play because Macbeth dies at the end of the play after a short period of glory. In this, you can compare him to a leaf.
Over the course of the play, Macbeth goes from being a relatively minor nobleman to being the king of Scotland. You can sort of compare this to the budding of a leaf, going from nothing to something relatively important. But then Macbeth declines. His kingship starts to fail and he is eventually killed. You can compare this to the decline of the leaf as autumn comes. The leaf withers and falls off the tree.
So I think you can see the leaf as a metaphor for how short Macbeth's "hour upon the stage" is in this play.