Magnified apples appear and disappear, Stem end and blossom end, And every fleck of russet showing clear.
No surprise here: he's going to dream about apples. The apples have an otherworldly appearance, as they are "magnified" in his mind. They float by him like little orbs or stars across his vision.
They "appear and disappear," similar to how the real apples would disappear into the barrel after he picked them. The speaker can see the brownish-reddish specks that dot the surface of the apples. This color is called "russet."
My instep arch not only keeps the ache, It keeps the pressure of a ladder-round. I feel the ladder sway as the boughs bend.
The speaker describes how realistic his dreams about the apple-picking are going to be. He can even feel the pressure of the ladder on the bottom of his foot, and the "swaying" of the ladder against the bending branches of the apple tree.
Frost zooms in on a tiny but telling detail: the speaker feels not only the ache from the ladder on his foot, which would be natural to feel later in the day. He also feels the continuing pressure of the ladder, as if he were still standing on it.
Since it's now clear that he is dreaming, we'll encourage you to look at again at the Biblical story of Jacob's ladder.
A "ladder-round" is one of the round steps on an old-fashioned wooden ladder.
And I keep hearing from the cellar bin The rumbling sound Of load on load of apples coming in.
The dream is so realistic that you feel like you're right there with him in the orchard. He hears the sound of barrels of apples being unloaded into a bin for sorting, and we practically can too.
The speaker is not the only apple picker: others are working in the orchard too. They constantly empty and refill their barrels throughout the day.