Paris is with Friar Lawrence arranging the marriage, when Juliet arrives.  When Paris leaves, Juliet asks the friar for a way out of the marriage, or she will kill herself.  Friar Lawrence has a plan:  He will give Juliet a potion, and she will drink it the night before she gets married.  She will look dead, and will be laid in a tomb, not buried, instead of being married to Paris.  She will wake up in the tomb, and be whisked away by Romeo to Mantua, a nearby city that Romeo was staying in.