What's meant by "GPS controlled tractors"? I know what GPS is but not clear how it is used in tractors; similar to how it's used in cars?

As far as tractors go, the primary function is localization for automatic steering and other function automation (raising an implement on the headlands, for instance), and, especially in older systems, simply being aware of where you have already crossed the field.

Once you attach an implement, then there are all kinds of other functions that it may be used for, like variable rate application. In harvesting applications, there are systems where the combine can take over control of the grain cart tractor while unloading on the go to make sure it stays in the right spot, again localized via GPS. The combine itself will also map the crop yields by its location, and of course automatic steering like the tractors have.

With RTK these tractors can achieve centimetre accuracy and repeatability which opens the door for all kinds of possibilities.

Plowing, planting, fertilizer and picking can be done within inches. A farmer can now plan on the contours of the land and the tractor can steer. This allows for higher yields and better land usage.

Not to say that plowing, etc isn't exciting work, but with the tractor doing most of the work, you can rock out to tunes in your AC environment. When they install the GPS steering, there is a free minifridge. :-)