Brutus has just completed his speech to the crowd explaining why he killed Julius Caesar. He has argued, of course, that Caesar was a tyrant, and that it was his duty as a senator of the Republic to protect Rome from tyrants:
If then that friend demand why Brutus rose against Caesar, this is my answer: Not that I loved Caesar less, but that I loved Rome more. Had you rather Caesar were living and die all slaves, than that Caesar were dead to live all freemen?
As he is finishing his speech, Mark Antony approaches, bearing Caesar's body. In a last rhetorical flourish, intended to remind the crowd of his commitment to Rome, Brutus says that
...as I slew my best lover for the
good of Rome, I have the same dagger for myself, when it shall please my country to need my death.
In other words, Brutus, who killed Caesar, acknowledges his willingness to kill himself if it would be what is best for Rome. He is reminding the listeners that his motives were pure in murdering Caesar, who, by his contempt for republican government, was the real criminal. In terms of the plot, he is, of course, foreshadowing his own suicide, which in fact occurs by his own hand when he stabs himself rather than being carried back to Rome after his military defeat.