In the famous balcony scene in act 2, scene 2, Juliet says, "What’s in a name? That which we call a rose / By any other word would smell as sweet." In other words, names, whether they're of flowers or human beings, are ultimately meaningless. Juliet is using flower imagery here to lament the absurdity of two young people, head over heels in love with each other, unable to be together because one of them is called Montague and the other Capulet.
In the following scene, Friar Lawrence is picking herbs for the strange potions and medicines he makes. He comments on their different qualities, observing how some are poisonous, whereas others are beneficial to health. Nevertheless, even the poisonous plants and herbs can be useful, so long as they're not abused.
The friar's musings foreshadow the tragic events that take place later on in the play, when Juliet will take one of his potions and fall into such a deep sleep that she will appear dead to everyone, including Romeo. Overcome with sorrow at what he thinks is Juliet's death, Romeo kills himself by taking poison. And when Juliet awakes from her drug-induced slumber and sees Romeo's dead body, she too takes her own life, stabbing herself to death with Romeo's dagger. The friar was right: there's nothing on earth that can not turn bad if it's abused. And sadly, the sleeping draught he prepared for Juliet definitely falls into that category.