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.
Real (single precision floating point value)
path_speed = -1 + random(2);
The above code will set the instance to travel the path at a random speed between -1 and 1.