Friar Laurence believes that both plants and men have attributes that can be both good and bad, depending on how they are used.  A plant with medicinal qualities can cure, but also can be poisonous if used incorrectly - "within the infant rind of this weak flower...poison hath residence and medicine power" (Act II, Scene 3, Lines 23-24).  By the same token, attributes in men such as love, for example, can be a wonderful thing, or, used incorrectly, can be destructive - "virtue turns to vice, being misapplied" (Line 21).  The Friar's words pertain to Romeo and Juliet, whose love leads to tragedy and death because it is not tempered with moderation.