You are indebted to someone in a way that is not solvable with simple currency. It is not about money, but you owe someone a favor and the value of the trait determines just how large this debt is. You can choose yourself who it is that you owe the favor to.
-1: Someone saved you from a highly embarrassing situation, such as not being able to pay for your food at the bar.
-2: Someone got you out of a sticky situation which would've ended with you being severely beaten.
-3: Someone has quite literally saved your life. Could be through medicine, or treating you after you had been mortally wounded and nursing you back to life.
-4: Someone has more or less given their life up in favor of you. The debt could therefore be to their next of kin, their family or similar. The person could also have taken blame and been put in prison instead of you, in which case they are still alive and this debt gnaws on your conscience.