I assume you are talking about these lines from Act V, Scene 1:
Come, cordial and not poison, go with meTo Juliet's grave; for there must I use thee.
It is not exactly what you quote, but it seems to be the only place in the play that the word "cordial" is used.
The reason it is called this is that Shakespeare was using the word "cordial" to mean a medicine. What Romeo is saying is that what the apothecary sold him is not a poison but rather a medicine. In other words, it is good for him. He is saying that if Juliet is dead, then poison is what he needs because all he wants to do is die with her.
Get Ahead with eNotes
Start your 48-hour free trial to access everything you need to rise to the top of the class. Enjoy expert answers and study guides ad-free and take your learning to the next level.
Already a member? Log in here.