Short Answer: Prince Escalus warns that anyone who fights in the Capulet/Montague feud again will be put to death.
Shakespeare opens his play with violent action, foreshadowing the violent love to come. This immediate action appeals especially to the groundlings, the people who stand in the central ground area beneath the tiered seats. Often they would call out and engage with the action.
In the first scene, after the servants of both the Montagues and the Capulets exchange insults and Benvolio fails in his attempt to quell the disturbance taking place, Tybalt of the Capulets enters, drawing his sword against Benvolio, exclaiming,
What, art thou drawn among these heartless hinds?
Turn thee, Benvolio, look upon thy death. (1.1.65-66)
These words and reactions clearly indicate the bellicose nature of Tybalt and presage his future actions. Then, once the patriarchs of the two feuding families enter the fray, the commotion is loud enough to draw the attention of the prince of Verona, Escalus. Interestingly, the Prince uses the same word-play in order to make his point to the feuding families:
Rebellious subjects, enemies to peace,
Prafaner of this neighbour-stained steel,
Will they not hear? What, ho!you men, you beasts....(1.1.80-83)
After ordering the men to throw down their swords, he reminds Lords Capulet and Montague that three-times that they have caused disturbances in Verona with their "civil brawls"; therefore, if they fight again, their actions will bring about their death:
If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace...(1.1.95-97)
Then, the Prince orders Capulet and Montague to accompany him to discuss further his judgment in this case.