You give me a bushel of apples and I give you an IOU. This IOU promises to pay back your bushel of apples, or something else to your liking, in the future. We make this trade, apples for IOU, because you have too many apples and I want apples today. We’re both better off. Apples get consumed that would otherwise be left to rot on the ground.
Why would I take your IOU? Because I trust you. Or you put up some collateral. Or I have legal recourse. Or it’s the Christian thing to do. Or a myriad of other reasons. Why would you give an IOU? Because your apples have not yet blossomed. Because you need apples today to sell apple cider tomorrow. Or you’re hungry and have no other means to get apples. Or a myriad of other reasons.