I know you can find it on the FSX disk and install it and download a patch for it from www.fsinsider.com
But what exactly is it used for? Do you create stuff with it?
Software Development Kit.
The SDK can be used to create add-on components for Flight Simulator X. These components can be new or replacement aircraft, instrumentation, missions, scenery, terrain, airport ground vehicles, airport runways and buildings, special effects, camera views, and many other elements of the simulated world.