If you are referring to the conversation between Friar John and Friar Lawrence in Act 5 Scene 2, you probably know that Friar Lawrence had tasked Friar John with a mission earlier in the play.
In this conversation, Friar Lawrence wants to know what Romeo's response is as soon as he hears Friar John's voice. Friar John basically tells Friar Lawrence that he was not able to deliver his(Friar Lawrence's) letter to Romeo. Friar John recounts his earlier efforts in trying to get another monk to accompany him to Mantua. However, as soon as he finds the monk, the authorities have both monks quarantined because of their suspicion that the monks have been exposed to the plague.
Due to this unfortunate development, Friar John finds himself unable to deliver the letter as promised. Friar Lawrence laments their bad luck, as the letter contains very important information for Romeo, warning him of Juliet's false death. At this point, Friar Lawrence does not realize that Romeo thinks his beloved is really dead.
To remedy matters, Friar Lawrence tells Friar John to get him a crowbar, as he intends to rescue Juliet himself. He reasons that Juliet will wake up in three hours and will be furious that Romeo has not been apprised of the truth. Friar Lawrence tells Friar John that he will keep Juliet with him until Romeo shows up. Meanwhile, he plans to write Romeo another letter.