In Act IV, Scene 1, Friar Laurence advises Juliet to drink a potion he gives her. He says that it will make her look and seem like she is dead. It will stop her heart, her breathing, and drain the color from her skin. He also says that the potion will only last for 42 hours and then she will wake "as from a pleasant sleep." Her family, thinking she is dead, will bury her in the Capulet tomb. Meanwhile, Friar Laurence, will send a letter to Romeo explaining that they faked Juliet's death. Romeo will meet Friar Laurence at the Capulet tomb and they will both stand gaurd and watch over her while she wakes from the potion. Finally, "Shall Romeo bear thee hence to Mantua. And this shall free thee from this present shame." In other words, Romeo shall take her to Mantua, where she will live as his wife.
Friar Laurence was certainly a reliabe person to devise this plan. As we see in Act II, Scene 3, when we first meet Friar Laurence, he is outside in his garden at dawn, picking both poisonous and healing herbs. We see this when he states, "Now, ere the sun advance his burning eye, the day to cheer and night's dank dew to dry, I must upfill this osier cage of ours with baleful weeds and precious-juiced flowers." He goes on further to describe mother nature as being full of powerful elements. Hence, we know from the beginning that Friar Laurence is an herbalist, so it is not a stretch to immagine that he could make up such a potion and suggest Juliet use it.