I believe a npc in each Residential Area sells it (the Estate Caretaker from what I remember), and it's an item that allows you to purchase a section of land (called a lot) for a house in a Residential Area. Unfortunately land is a limited resource and vacant lots have since all sold out on most servers (and especially so on Balmung).

There is a land reclamation function that was added to the game though where if a house's owner (either a FC or a single player) has not used their house in a certain amount of time (about 45 days if I remember right), the house and land is automatically repossessed, the house demolished and the land put up for sale again. Stormblood will apparently be getting a new residential area too for Ishgard so that will increase the availability of houses hopefully.