In his funeral oration to the crowd of common Romans, Brutus explains that the assassins killed Julius Caesar
because he become too ambitious and threatened to turn into a tyrant. Brutus says that although he loved Caesar, he loved Rome more, and he did what he had to do to protect his city. By the end of his speech, the crowd is swayed in favor of him.
Antony, however, speaks next. He has promised to say nothing evil of Brutus or the conspirators, so he uses mockery to turn the crowd against them.
Antony asserts that Caesar was not an ambitious man. He says that Caesar used the spoils of war for the general benefit of Rome. Antony also states that Caesar sympathized deeply with the poor, saying:
When that the poor have cried, Caesar hath wept.
Ambition should be made of sterner stuff.
In other words, if Caesar were truly ambitious, he would not have shed tears for the poor, as there was no power to be gained that way.
Finally, Antony says that he offered Caesar the crown three times and that in every case, Caesar refused it. That is not the act of an ambitious man.
Antony mocks Brutus, saying that if he thought Caesar was ambitious it must be true because he is an honorable man. Antony repeats the phrase about Brutus being an honorable man so many times that it becomes clear to the listeners he is speaking acidly and ironically—he does not think murdering Caesar was in any way honorable. Antony's speech manages to turn the crowd against Brutus and the conspirators.