It's one of my favourite moments in all of Shakespeare, though that's not its significance in any wider dramatic sense! After Caesar has been assassinated, the conspirators are bathing their arms in Caesar's blood. Cassius suddenly realises what an epic moment this is:
How many ages hence
Shall this our lofty scene be acted over
In states unborn and accents yet unknown!
How many times shall Caesar bleed in sport...
So oft as that shall be,
So often shall the knot of us be call'd
The men that gave their country liberty.
Cassius imagines his own - and the conspiracy's - actions being renacted by plays later: obviously, quite an ironic thing for Shakespeare to wrote, bearing in mind that "Julius Caesar" is a play written about a true event. So you have a real event imagining itself as a play set within a play written about a real event!
What's absolutely key to this moment is the fact that Cassius thinks that history will look upon them with unambiguous smiles: they will be "the men that gave their country liberty". Yet, in Shakespeare's day, and now - that isn't true. "Julius Caesar" never tells us whether the assassination was a good or a bad thing: and modern commentators disagree about whether Caesar needed to be killed. For me, the significance is simple: it's one confident angle on what is really a horribly interpretable situation.
The context of these words of Cassius in "Julius Caesar" is after the assassination of Caesar. Brutus suggests that all the assassins "bathe" their hands in the blood of Caesar up the the elbow and besmear their swords; then, they should walk out "even to the market place," waving their arms over their heads crying, "Peace, freedom, and liberty!"
As they stoop to wipe the blood of the fallen ruler upon their arms, Brutus asks them to imagine how many times this act will be recreated in plays, and Cassius responds that the act will be replayed as long as he and the other assassins are lauded as the men who freed Rome from tyranny ("The mean that gave their country liberty").
Ironically, their hopes of being considered the men who freed Rome from tyranny is extremely short-lived as Antony in beautiful rhetoric sways the crowd against the conspirators in his oration. For, Antony turns and berates them and their "liberating gesture":
Who else must be let blood, who else is rank/If I myself, there is no hour so fit/As Caesar's death's hour, nor no instrument/Of half that worth as those your swords, made rich/With the most noble blood of all this world