This is an in line filter that you run in the tubing between the pump and the reservoir in order to stop dirt and other debris being spread through and blocking the nozzles and systems. Also decreases likelihood of spraying the enclosure with debris.
Here's what the manufacturer says about their product:
MistKing 10 Micron Sediment Filter
- Brand: MistKing
- Product Code: 10M-Filter
- Availability: In Stock
- Ex Tax: £25.00