The speaker wants to be clear here, so he's going to elaborate on what he was talking about in Line 1. Just like nature's first green is gold, her first leaf is a flower. In spring, trees and bushes bloom with gorgeous flowers, which are replaced by green leaves in the summer.
Frost is really getting into his poetic groove here, when he pops a metaphor into this line. The first flowers of spring aren't actually leaves in disguise; the speaker is using figurative language to intentionally blur the line between flowers and leaves. Eventually, in real life, the blooms die and drop off the trees, making room for the leaves, which come to soak in nourishment from the sun.