Gittin 37 - Benefit of both rich and poor
Prosbul is related to the Greek prosbuleibutei (a council to protect people from harm) and means "benefit or rich and poor". Rich, because they do not transgress "do not refrain from helping with a loan", and poor, because it helps them obtain a loan.
Prosbul works by assigning the loan to court for collection. It is then "as if collected", and such loan is not canceled by the 7th year.
Only unsecured loans are canceled and thus require prosbul.
Even if a debt is canceled by the 7th year, it is meritorious to return the money as a gift.