From Academic Kids
A plat is a map, drawn to scale, showing how a piece of land is divided into lots with streets and alleys, usually for the purpose of selling the described lots; this is known as subdivision. After a plat is filed, legal descriptions can refer to lot numbers rather than portions of sections. Plats can also legally dedicate land for road and other rights-of-way.
The creation of a plat map is also an important step in the process of incorporating a town or city according to United States law. Because the process of incorporation must be done at a courthouse, the incorporation papers for many American cities may be stored hundred of miles away in another state.
For example, the original plat for the city of San Francisco, California, filed in 1849, is kept at the Clackamas County courthouse in Oregon City, Oregon, then the capital of Oregon Territory and the site of closest federal land office. This is because California did not become part of the United States until 1850.