The Ceres Field is a region in which crops grow twice as fast.
To access it, you'll first need to unlock the Cartographer, 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:
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.
To unlock the Ceres Field, you'll need to complete a number of prerequisite tasks and pay to unlock the actual region.
You can see your progress for the prerequisite tasks by clicking on the "Destinations" tab after clicking on the region on the map. They are:
Have farm upgrade #6
Produce 500 Maple Furniture
Produce 500 Goat's Milk
Once you have fulfilled the prerequisites, you'll be able to unlock the glade for 1,500,000 Thalers or 100 Gold Bars. You can skip the prerequisites for Gold Bars, but this won't grant you the products involved, and you'll still need to pay to unlock the region.
The Ceres Field starts 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.