You can use this variable to get or to set the speed of a path after it has been started. The speed is the number of pixels along the path that an instance should move each step, when the path node has a speed factor of 100. You can use negative values to signify that the instance should follow the path in reverse.
For functions relating to paths, see here.
path_speed = -1 + random(2);
The above code will set the instance to travel the path at a random speed between -1 and 1.