The Cartographer is a building that allows to access a number of regions that specialize in certain aspects of farm life and server as expansions of sorts to your main farm area. You can find it to the west of your farm:
Some of the regions you can unlock for the Cartographer are:
Diana's Glade, a forest clearing where your farm animals give 20% more products
The Sea of Flowers, which allows you to grow a number of flowers, which can be sold directly or processed into lovely bouquests
The Ceres Field, a field which lets crops ripen in only half the usual time
The Floodplains, a landscape of ponds and lakes perfectly suited to raising and catching fish, and farming reeds and basket willow
The Shadow Weald, a dark forest which lets you grow blueberries, beech trees and pinewood mushrooms
These new regions start out with 25 plots which are ready for use immediately; additional plots can be cleared by removing obstacles (regular obstacles and special ones which can only be cleared for Gold Bars) and expanding the field, similar to how it works on the main farm.
The Cartographer is available to players level 40 and up for 15,000,000 Thalers or at level 15 and up for 150 Gold Bars. Individual regions (with the exception of the castle, which doesn't need to be unlocked and is accessible without unlocking the Cartographer) will need to be unlocked by completing a number of prerequisite tasks and paying to unlock the actual region. You can see what these are by clicking on the "Destinations" tab after clicking on the region on the map.