It's two things. fspy is an external program: https://fspy.io
fspy has an importer for Blender: https://github.com/stuffmatic/fSpy-Blender
fspy helps getting the perspective of the image down so that, say, a photo of a tall row of apartment buildings taken at an angle can be properly applied as a texture to a building in Blender.
One can do that one's self to some extent just by lining up the UV map with the image. With a few loops cut into the cube the UV will seem less distorted. FSpy just helps with the process.