What is the meaning of the expression "that's apples and oranges."?

A male colleague of mine at work is always using the expression "that's apples and oranges," or "you're comparing apples and oranges." This guy is an engineer and has a reputation as being a logical thinker. I don't have the nerve to ask him what this expression means. Can someone... show more A male colleague of mine at work is always using the expression "that's apples and oranges," or "you're comparing apples and oranges." This guy is an engineer and has a reputation as being a logical thinker. I don't have the nerve to ask him what this expression means. Can someone please explain it to me.
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