The noun “fleck” means a mark or spot; the adjective “russet” means brownish or reddish in color. The apples in the speaker’s dream are almost overwhelming in their size, for they are so large that “every fleck of russet” in their flesh is easy to see.
The adjective “hoary” means grayish or white, especially in reference to something old—such as an aging person’s hair. Frost’s depiction of a “world of hoary grass” indicates that winter, which represents death, is close at hand.
The noun “essence” refers to the basic, intrinsic quality of something, especially something abstract. An essence can also be a fragrance. So, Frost means that the speaker can smell “winter sleep,” which has the “scent of apples,” as he begins to fall asleep and dream.
