What does friar laurence explain about the nature of these herbs?

"In herbs, plants, stones, and their true qualities:

For nought so vile that on the earth doth live

But to the earth some special good doth give,

Nor aught so good but strain'd from that fair use

Revolts from true birth, stumbling on abuse:

Virtue itself turns vice, being misapplied;

And vice sometimes by action dignified.

Within the infant rind of this small flower

Poison hath residence and medicine power"

Friar Laurence is discussing the duality of human nature by saying there is nothing that does not have a good and bad quality.