Friar Lawrence's speech, considered a soliloquy because he is speaking at length while alone on stage, at the beginning of Act II, Scene 3, serves three purposes.
First, in Shakespearean theaters no lighting or props were used so the language had to set the scene. In the opening lines the Friar uses both personification and a simile to inform the audience it is morning:
The gray-eyed morn smiles on the frowning night,
Check’ring the eastern clouds with streaks of light,
And fleckled darkness like a drunkard reels
From forth day’s path and Titan’s fiery wheels.
In fact, it is the morning after Romeo and Juliet
first meet and when Romeo
shows up we know he has been up all night.
Second, and to directly answer your question, the Friar is telling us that he is something of a chemist who can concoct both medicine and poisons. While he is gathering flowers and weeds he comments:
Within the infant rind of this small flower
Poison hath residence and medicine power.
For this, being smelt, with that part cheers each part;
Being tasted, stays all senses with the heart.
The fact that he knows about medicines and poisons foreshadows his plot later in the play for Juliet
to drink a sleeping potion and fake her death so that she will not have to marry Count Paris
and will eventually be reunited with Romeo. From this speech we know he is capable of mixing such a brew. Unfortunately, ...well, you know the rest of the story.
Third, Shakespeare is commenting on the nature of human beings. In this play, people who are essentially good (there is not a truly evil character in the play--at least not as evil as some of Shakespeare's true villains like Iago or Richard III
) are led to tragedy
. Shakespeare says that, like the weeds and flowers, every human being may display elements of both good and evil. The Friar says,
Two such opposèd kings encamp them still
In man as well as herbs—grace and rude will;
And where the worser is predominant,
Full soon the canker death eats up that plant.