Once you have the necessary levels, she will offer to take you on as a partner to rebuild her Manor. She will then begin to teach you how Player-Owned Farms work. At the end of her explanation, you will be be set free to run the farm as you see fit; within the scope of what it allows of course.
That is not the end of your interactions with Granny P, however, you will need to access the Farmers' Market through her. This will provide you with access to some species to raise, farm upgrades, and various other awesome perks to purchase.
You can also speak with her about her family or ask her to tell you a joke. Some of the things she says will change regularly. Her trusty companion, Sam the Sheepdog, can also be found nearby.