When you're about to be tested on a quote, what you want to do first is ensure you understand the point of the scene itself: what is happening and who is involved. In this case, we have Marullus and Flavius meeting a group of citizens who have closed their shops for the day in order to celebrate the return of Caesar. Caesar has just defeated Pompey--the last of his fellow triumvirates--in battle and even pursued and defeated his sons (to secure his power). Marullus and Flavius are fairly certain that Caesar will now become a tyrant, and they want to avoid that if possible. This is why they tell the citizens to go back to work, then they go around removing all the decorations on Caesar's statues. They don't want Caesar to think it will be easy to take absolute control.
There is some punny bantering between them and a couple of trademen (this is trademark Shakespeare stuff), but the first important quote--which tells us what Marullus and Flavius are thinking and why--occurs with Marullus:
Wherefore rejoice? What conquest brings he home?
What tributaries follow him to Rome,
To grace in captive bonds his chariot wheels?
You blocks, you stones, you worse than senseless things!
O you hard hearts, you cruel men of Rome,
Knew you not Pompey? Many a time and oft
Have you climb’d up to walls and battlements,
To towers and windows, yea, to chimney tops,
Your infants in your arms, and there have sat
The livelong day with patient expectation
To see great Pompey pass the streets of Rome.
And when you saw his chariot but appear,
Have you not made an universal shout
That Tiber trembled underneath her banks
To hear the replication of your sounds
Made in her concave shores?
And do you now put on your best attire?
And do you now cull out a holiday?
And do you now strew flowers in his way
That comes in triumph over Pompey’s blood?
Run to your houses, fall upon your knees,
Pray to the gods to intermit the plague
That needs must light on this ingratitude.
In essence, Marullus is arguing that the same people who are presumably here to celebrate Caesar's victory were the same who would go to the tops of the ramparts and wait patiently all day to see Pompey even appear in the streets, whereupon their their shouts would be so great that they echoed. He's calling them hypocrites, and says they should be ashamed of themselves. This quote is important because we (the audience) need to understand the setting of the play, the situation (or "argument"), and where Marullus and Flavius stand in relation to Caesar.
The second important quote in this scene is from Flavius, after he's reassured Marullus that they can and should confront the citizens and send them home and remove the ornaments from Caesar's statues:
These growing feathers pluck’d from Caesar’s wing
Will make him fly an ordinary pitch,
Who else would soar above the view of men,
And keep us all in servile fearfulness.
Their motives are clarified here: Don't give Caesar any reason to believe he has any more power than he already believes he does. The triumvirate was created in order to avoid tyranny, and while it didn't work out as planned, their hatred of tyranny is no less strong.