WARNING: The following is book knowledge only, I've never done any of this myself.
DVC, Disney vacation Club, is a time share situation. When someone buys a DVC time share, what they are really buying is a certain number of points per year for a certain number of years. The minimum number of points that a new DVC owner can buy from DVC is 150 points per year. The number of years is non-negotiable, it is already pre-set, most DVC properties until some time in the 2040s and SSR until some time in the 2050s.
So a DVC owner doesn't buy one week per year of use or two weeks per year of use, they buy a certain number of points. How they use those points is, with a lot of exceptions of course, up to them.
So let's say a DVC owner wants to spend a week at Old Key West in a studio room. For a week in value season (like mid-September), it will "cost" 8 points per day for weekdays and 20 points per day for weekends, so a week will "cost" 80 points. For a week in holiday season, December 25 through January 1, the "hottest" week of the year, it will "cost" 15 points per day for weekdays and 36 points per day for weekends, so that week will "cost" 147 points. If you think about it, it makes perfect sense -- those of us who pay cash to stay at the resorts know it costs more to stay there at the more popular times versus the emptier times.
So, Disney thinks it's perfectly cool and legitimate for DVC owners to rent out their points to non-DVC folks like us. It's a private transaction between you and the DVC owner. You can find DVC owners willing to rent points in several places, including DISboards. You and the DVC owner agree on the price per point. You pay the money to the owner, not to Disney. The owner alerts Disney of the transaction, so they know it'll be you staying at WDW during that time and not the owner himself/herself.
So let's say I bought into DVC and I paid for 250 points per year. Let's say I use only 100 points this year, or let's say I use none. I may want to rent out my excess points and make some money. It makes me happy because I can get some extra cash, and it may make you happy because, generally speaking, the amount of money you'd pay to rent DVC points is less than you would pay Disney to stay at that exact same resort or a resort with similar amenities.
There are MANY details I've left out here, but now you've got the main idea ... I hope.