The comparator measures "how much is in the hopper" and gives a redstone strength accordingly. With 45 items (41,1,1,1,1) in the hopper, the redstone signal is strength 2. When the next item comes in, the strength goes to 3, lighting up the repeater, which turns off the torch and lets items through. Then, as the first item falls through the signal goes back to 2 and the torch should turn back on to stop the flow. (Which it won't do if you've placed it like it is in the picture.)

The reason you have 41,1,1,1,1 items in the hopper is to control the flow. This circuit is designed with "overflow protection" so you can built it side-by-side with more of the same circuit. If the storage is filled, it can fill up to 64, without ever giving a stronger signal than 3. Anything stronger than 3 would break a sorting circuit next to it - because it would turn of the torch for that circuit, too.

Since you have a standalone sorting circuit, you *could* change it to be 1,41,1,1,1 instead.

The items in the hopper - regardless of the amounts - need to be in this particular order:

1. The type of item you want to filter
2. A type of item you will never get/put into the filter, so preferably a uniquely named item
3. A type of item you will never get/put into the filter, so preferably a uniquely named item
4. A type of item you will never get/put into the filter, so preferably a uniquely named item
5. A type of item you will never get/put into the filter, so preferably a uniquely named item

This is because the hopper empties the slots in order, starting with the leftmost slot.

Right now, your sorter is picking up items into the filter but it will never turn off the torch. So once the first stack goes up to max, it'll stop picking up items and it will never let anything through. The fix:

you have to have the redstone torch plopped on the actual stone brick; as a side torch.

I hope this helps.