The apple tree, apples, ladder and buckets around the tree is the literal meaning. The metaphoric meaning is much deeper though.
Metaphorically the apples are his desires, the tree is his life, the ladder is a staircase to heaven and the long sleep is death.
Some people desire money, love or happiness. All people have desires.
His desires are always popping up and coming into his head just like the apples are, but he will not be able to accomplish all of them.
In the poem it says that the arch of his foot hurts from being on the ladder. If the ladder is like a staircase to heaven then he could be holding on to life because he has been on the ladder for so long.
Another literal meaning can be that sometimes you get tired of doing things. The author has been apple picking so many times that he is tired of it.