In his first speech in Act 1, Scene 1, Prince Escalus actually gives two warnings to the feuding peace breakers. Before I discuss the actual warning you are asking about, it can be helpful to discuss the first warning that leads up to the second.
His first warning is given as an attempt to break up the current fight. We see this in his lines,
On pain of torture, from those bloody hands
Throw your mistempered weapons to the ground. (81-82)
Prince Escalus is warning the miscreants that he will punish them by torture if they don't cease fighting immediately. He is commanding them to throw down their swords, which he refers to as "mistempered weapons," meaning "misused" weapons. He is calling their weapons improperly used because the miscreants are using them to fight and kill their neighbors when they should be using them to fight and kill their enemies instead. The harshness of the punishment and the passion in his language help us to see just how strongly Prince Escalus feels about this issue. Hence, Prince Escalus's first warning is a warning of punishment given to stop their present fight and shows us Prince Escalus's feelings about the feud.
The second warning Prince Escalus gives is intended to punish anyone of the family members who start a public battle again. His second warning is seen in the lines,
If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace. (92-93)
In these lines, he is warning the feuding family members that he will issue capital punishment if they begin another public brawl. At this point, the two families have caused three public brawls due to some trivial comment, and the Prince is sick of it, as seen in the lines,
Three civil brawls, bred of an airy word
By thee, old Capulet, and Montague, Have thrice disturb'd the quiet of our streets. (85-87)
Hence, the Prince gives two warnings in the first act, the first is a warning of punishment given to break up the fight, and the second is given to issue capital punishment (death) if the miscreants continue to battle.
"If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace."
The families have already caused three civil brawls because of their feud. The prince threatens death for whoever causes the fourth.