It's an accumulation of credit towards the home rent component. So if you have home rent at $500 and you're doing a 50% credit then the tenant will get $250 credits per month towards a home in the Park. Once they have enough credits they purchase the home they're in or one of the other homes available.
The common comparison is to Southwest points. You can use the points on various flights to different destinations, and depending on the cost and demand the number of credits varies accordingly.