Paris, one of the Capulets, is with Friar Laurence, and he has come to announce first to Friar Laurence and then to Juliet herself (when she enters) that he is to marry Juliet next Thursday. Friar Laurence understandably protests that this is a very abrupt arrangement, but Paris' answer is decisive:

My father Capulet will have it so,
And I am nothing slow to slack his haste.

Ironically, it has been the outward signs of grief that Juliet has displayed that have made the Capulets feel that a hasty marriage is in her own best interests:

Now, sir, her father counts it dangerous
That she do give her sorrow so much sway,
And in his wisdom hastes our marriage
To stop the inundation of her tears,
Which, too much minded by herself alone,
May be put from her by society.
Now do you know the reason of this haste.

Thus, Paris serves two functions here in advancing the plot of Romeo and Juliet. He announces a new development, his own proposed marriage to Juliet, which is full of dramatic irony, seeing that the audience knows that Juliet is already married to Romeo. Moreover, this development heightens the dramatic tension greatly, since it gives Juliet and Friar Laurence only a few days to find a way out of the dilemma and thwart the marriage of Juliet to Paris, which would not only be undesired but also bigamous.