In the beginning of this scene Friar Lawrence is giving advice to Paris about marrying Juliet. Juliet later enters the scene to talk to Friar but meets Paris as well. Paris leaves the scene excited to marry Juliet on Thursday, but Juliet does not want to marry him. Friar suggests a solution that will make Juliet not marry Paris; he gives her a sleep potion that last 42 hours. This scene is important because Friar Lawrence's solution creates problems in the future scenes.
In this scene, Juliet comes back from Friar Lawrence's lair and asks Capulet for forgiveness while on her knees. Capulet forgives Juliet and becomes convinced that she wants to marry Paris; he sets the wedding date to Wednesday. This scene is important because it establishes that Capulet and Juliet's relationship is back to normal again. Also because Capulet's decision to have the wedding early causes problems with Friar's solution.
In the scene Juliet is contemplating rather to drink the vial or not. Juliet wonders if the potion will put her to sleep, kill her, or make her hallucinate. This scene is important because it shows Juliet's independence and maturity. Also because she drank the potion, Friar Lawrence's problematic solution proceeded.
In this scene the nurse discovers Juliet asleep and believes she is dead. As everyone comes into the room an overwhelming feeling of melancholy fills the room. This scene is important because it illustrates Friar's plan.