What subject are Lord Capulet and Paris discussing in lines 1-5 of Act 1,Scene 2?
In Shakespeare's Romeo and Juliet, Act 1.2.1-5, Lord Capulet tells Paris that Lord Montague is required to bring an end to public disturbances caused by the feud between the two families, just as he, himself, is. The most significant part of these five lines comes next, however:
...'tis not hard I think
For men so old as we to keep the peace.
In other words, it shouldn't be too hard for the two men to keep their families from fighting in public. Of course, this turns out to be very hard. This line contributes to the idea of the feud, and serves as a bit of foreshadowing.
Paris contributes to the conversation by stating that this, indeed, shouldn't be too difficult since both men have a reputation of being honorable. One could interpret this comment as direct characterization, or as Paris flattering Capulet in order to further his "suit," which he asks about in line six of the scene.
Finally, the fifth line, again spoken by Paris, adds background to the feud, as Paris says that it is a pity that the two families have been fighting for "so long."
What they are discussing at the beginning of this scene is the fact that Lord Capulet and Lord Montague have been forced by the Prince to swear not to fight anymore.
In Act I, Scene 1, the servants of the two families were fighting in the streets. The Prince came and was very angry. At that point, he tells Lord Capulet to come with him right away. He tells Montague to come to the court later in the day. He says he will tell them what his judgement is.
We can infer from what Capulet says to Paris that the Prince has told both lords that they have to keep the peace.