If something is affordable, it's priced reasonably, and you have enough money to buy it. Your friends might be jealous that the bike you bought at a yard sale was so affordable.
If you live in an affordable city, it means that rents aren't too high for most of its residents, and if you buy an affordable dinner for yourself, it doesn't cost too much. The adjective affordable can either mean "cheap," or it can imply that even if it's expensive, you have enough money to easily buy it. The verb afford is at the root, and its earliest meaning was "accomplish." Gradually, afford came to have the meaning "manage to buy."