Great literature is replete with metaphors and similes which, by making unsual comparisons between ideas and things, enable readers understandings of life.
In Scene 2 of Act IV, when the murderers enter, they are insulted by Macduff's son, and one stabs the boys saying, "Young fry of treachery," a metaphor for the boy's being the son of a man who is a traitor. In the following scene, Macduff's first words contain two similes [comparisons of unlike things using the words as or like]:
Let us rather
Hold fast the mortal sword, and like good men
Bestride our down-fall'n birthdom.
Each new morn New widows howl, new orphans cry, new sorrows
Strike heaven on the face, that it resounds
As if it felt with Scotland and yelled out
Like a syllable of dolor. (IV,iii,3-9)
Then, in line 22 of this scene, Malcolm says,
Though all things foul would wear the brows of grace,
Yet grace must still look so. (IV,iii,22)
This line means that his thoughts cannot change what a person is; things may look good, but be filthy; nevertheless, good must still resemble them when they are disguised. "The brows of grace" is the appearance of seeming good.
Later in the scene, Malcolm uses a simile as he says,
Will seem as pure as snow, and the poor state
Esteem him as a lamb, being compared
With my confineless harms. (IV,iii, 59-61)
Further in this scene, Malcom employs a metaphor as he says, "Pour the sweet milk of concord into hell" as he states that he would cause discord in the world, destroying peace.