Filter wool is an essential media that should be added to all types of filters but especially external filters. It's the last layer (i.e. the top one) and traps any fine particles that would normally make their way into your aquarium.
It's really important to use a lot of this in your filter as it works properly when sufficient is used, so pack it in as much as possible. You can probably get a few uses out of it before it needs chucking out and replacing with fresh wool.I recommend keeping it in your filter for maybe 3 weeks. When you clean it and it doesn't bounce back to the original shape, you know it's time to replace.
If you run 2 filters, I recommend loading one of them up with only filter wool - you will be amazed how clean it will make your water and the polishing effect it has.