Willy here is reproaching Howard, his boss. Willy has worked hard for Howard's company all his life. He has done his best for this company; they have taken his best years, plucked the 'fruit' of his youth, so to speak, but now that he's old and can't compete with the other salesmen, they're just casting him off in the way that one might discard fruit peel. It is a very telling quote; Wily feels he's had no reward whatsoever for all his years of dedicated service. Willy feels that times have changed and there is no personal warmth in the workplace any more, that it's all become too competitive and commercial. Willy recalls his friendship with Howard's father Frank but emotional appeals get him nowhere with Howard.
Willie's choice of a metaphor is a poor one. It either shows a flaw in Arthur Miller's writing or else Miller was intentionally making Willie look unintelligent and unimaginative. Eating an orange and throwing the peel away is exactly what you can do and what is generally done with oranges and orange peels. What Willie was probably trying to say is something like this:
You can't use a man up and then throw him away the way you eat an orange and throw away the peel.
Whether it was Arthur Miller's mistake or intention, the metaphor, or simile, or analogy, or whatever it is supposed to be, is almost comical in its ineptness.
This metaphor, however inept, is at the heart of Arthur Miller's thesis. Under capitalism, people are used up and discarded without any consideration for their humanity, welfare, or personal feeliings.