It has a few uses. It can refer to the time that something must be done. For example: "The homework is due Friday." "The book is due (back) in a week." The "back" can be used if you're talking about returning (i.e. taking BACK) a book to a library. "Due to" means "as a result of" or "because". For example: "He doesn't go outside due to his allergies." "I can't return the book due to the fact that I set it on fire." "In due time" is an idiomatic phrase meaning "in a reasonable amount of time; not soon necessarily, but not so late as to be a problem." For example: "We'll get home in due time." "When are you going to return the book?" "In due time. Don't worry."

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