DHCP option 43 is one of the methods that can be used to tell the AP where the controller is so they can join. Since the AP's come out of the box with no configuration they don't know where the controller is. There are a few methods that can be used such as option 43, DNS resolution of cisco-lwapp-controller, broadcast, and OTOP. All you are trying to accomplish with one of these methods is to let the AP know where one of the controllers are. After it finds one it will send a discovery request to every controller in the mobility group, build a list of available controllers, then choose one to send its join request to.
Here's a link to the deployment guide section which describes the discovery and join process:
http://www.cisco.com/en/US/products/ps6366/prod_technical_reference09186a00806cfa96.html#wp1081382
