The below is when you want to do something when a sensor has been triggered within a particular range. This could be for predicting collisions between moving objects, and may want to trigger an avoiding action, or stopping the GameObject before a collision is made.
It's important that the rayStartPosition and the direction argument in the second parameter both originate from the same position, in this case rayStartPosition.
This fires a raycast from rayStartPosition in the forward direction for a length of collisionSensorLength - 50 Unity units. (50 meters)