The right side is where people farm and the left is more ranching. Obviously, there’s farms everywhere, but out west, it has to be irrigated, which can only be done cost effectively in certain locations.

If you know where to look, you can even make out rivers. The Platte river crosses the great plains to Denver (the big blue area).

In case you aren’t American, that line is the Great Plains. It’s basically the dividing line between where it historically gets enough rain to farm, and where it’s dry enough that farming gives way to ranching.