| Quote #1
Northumberland sure likes his similes. (A simile is a comparison of one thing directly to another.) Here, Northumberland says that civil warfare is like a horse that's broken out of its stall. In other words, the times are wild and unpredictable.
| Quote #2
Lord Bardolph is reporting false information to Northumberland here. The truth is that Hotspur has been killed at the battle at Shrewsbury, where the king's forces have been victorious. Consequently, the remaining rebels proceed with more caution in Henry IV Part 2, planning each of their steps carefully instead of running headlong into battle like Hotspur did in Part 1.
| Quote #3
When Northumberland learns that his son is dead, he calls for a major battle to "end" the "lingering" rebellion against the king once and for all. When he says let the "spirit of the first-born Cain / Reign in all bosoms," he imagines an entire country fighting and murdering their "brothers" until everyone is dead and buried in "darkness." Harkening all the way back to the first son's murder of his brother in the book of Genesis, Northumberland anticipates an apocalypse (end of the world).
We also notice that Northumberland uses a theater metaphor here when he says the world should stop being a "stage" where civil strife is dragged out in prolonged action (like a really long play). Instead, he wants the "rude scene" to "end." When we think about it, the metaphor conjures a vivid image of corpses scattered all over a theater stage, which is exactly what a stage looks like at the end of a tragedy. For funzies, you might to compare this passage to the end of Hamlet.