A metaphor is a comparison of two things to show the particular quality of one of those things. In Shakespeare's Julius Caesar Act I, there are a few important metaphors.
In Scene 1, the tribunes Marullus and Flavius are upset at the working men who have come out to cheer the return of Caesar. The tribunes were followers of Pompey who was defeated by Caesar. In line 36, Marullus compares the crowd to inanimate objects:
You blocks, you stones, you worse than senseless things
And later in the scene, Flavius compares Caesar to a hawk who might fly too high and keep the people in fear and servitude. He says,
Who else would soar above the view of men
And keep us all in servile fearfulness.
In Scene 2, Cassius tries to convince Brutus that Caesar has grown too powerful and it is up to them to keep Rome a republic and not a dictatorship. He compares Caesar to a carnivore and the people of Rome his meat. Cassius says,
Upon what meat doth this our Caesar feed,
That he is grown so great?
A little later in that scene, Caesar, sensing the possible tyranny of Cassius, compares him to a wolf or coyote. Caesar says,
Yond Cassius has a lean and hungry look;
He thinks too much: such men are dangerous.
In Scene 3, Casca uses an extended metaphor to say that Rome is on the verge of some terrible calamity. He comments on the severity of the weather and says he's seen a man with his hands on fire but the flesh not burning. He met a lion, but the beast ignored him and he saw an owl outside in the daytime. He believes these natural phenomena are an omen of something awful to come. He says,
"These are their reasons, they are natural,"
For I believe they are portentous things
Unto the climate that they point upon.
Later in Scene 3, Cassius again compares Caesar to a carnivore and the people of Rome his food. He says,
Poor man! I know he would not be a wolf
But that he sees the Romans are but sheep.
He were no lion were not Romans hinds.