This variable holds the current orientation of the path. When a path is created, its orientation is the default 0 degrees, but you can set this value to anything you wish using this. Remember that in GameMaker Studio 2 (unless you are using physics) the angles are calculated counter-clockwise, so setting the path orientation to 90° would rotate the path to the left. For functions relating to paths, see here.
The above code duplicates a random, pre-made path resource into the variable "mypath". This new path is then rotated 90°.