A rentier is a person who makes money based on what they own versus what they do.

In many cases, the item owned does not significantly depreciate in value, so the income is essentially money for nothing.  When the income thus acquired can be used to buy more of the original item, then the growth of wealth can easily be exponential.

If the item being rented exists in finite quantities, rentiers can create scarcity in order to drive up prices.  Meanwhile, since ownership is not transferred, the consumer gains much less than in a purchase transaction, so social inequities tend to be more skewed in rentier economies.