The plants that the friar is collecting on the morning Romeo comes to see him have both beneficial and detrimental applications; in other words, they have the power to be both good and bad for the people who use them. He says to himself, as if pondering the contradiction,

Within the infant rind of this weak 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. (2.3.23-26)

He says that this one particular flower that he collects contains both poison and medicine within it. If a person smells the flower, it can have medicinal and beneficial effects, improving the person's health; however, if a person tastes it, it can actually stop their heart and kill them, cutting off all of their senses. Friar Lawrence says that humankind contains the same opposing forces: we can be both good and bad, and when the bad outweighs the good, a person can be consumed by it.

Shakespeare's portrayal of Friar Lawrence in "Romeo and Juliet" is often a depiction of the corruption in the Catholic Church.  For, the ordained "man of God" takes several decisions upon himself that are contrary to doctrine.  For one thing, he delves in herbs and chemistry, which at the time, would not be an area in which a priest should be experimenting. To connote this "straying from the faith" Shakespeare has the good friar commenting in pagan faship upon the morning

From forth day's path and Titan's fiery wheels,/Now, ere the sun advance his burning eye, (II,iii,4-5)

In his basket, the priest has herbs and poison weed.  The friar exclaims,

Oh, mickle is the powerful grace that lies/In herbs, plants, stones, and their true qualities (II,iii,8-9).

Interestingly, it is with these items in his basket that the friar conjures a magical potion that causes Juliet to seem dead for a time; it is with these herbs and "poisons" that the friar would play God and alter the course of events.