Friar Laurence addresses the audience, and tells them about what he has to do that morning:
I must up-fill this osier cage of ours
With baleful weeds and precious-juiced flowers.
Friar Laurence has a basket (an "osier cage") which he's going to fill up with poisonous weeds and flowers with precious juices.
The earth that's nature's mother is her tomb.
What is her burying grave, that is her womb;
And from her womb children of divers kind
We sucking on her natural bosom find;
Many for many virtues excellent,
None but for some, and yet all different.
The earth, Friar Laurence continues, both gives life and removes life. And because of this dual nature of the earth, the flowers we get from the earth (her "children") tend to be of different and various kinds. Some are poisonous, some are good. They're opposites. And it's both of those kinds of flowers which Friar Laurence is collecting.