Basically, if the end light is on, it is in subtraction mode, and if tje end light is off, it is in addition mode. If you have a redstone input with a power of 6 in the back and an input of 4 on the side, the output would be either 2 or 10 dependant on the mode. Additionally, the highest strength it would output is 15. Lastly, if you place the comparator in front of a container of sorts, it will detect when items are in the container, and output a certain redstone strength dependant on the fullness of the container. I hope that helps.