Cheese cloth bag, that makes dry hopping or boil additions easier to deal with. For dry hopping tie bag loosely to allow maximum flow of beer through your hops and a better flavour extraction. Can also be used for adding fruit, coriander or spices to beers or ciders.
Partial mash brewers often use these for steeping grains too.
Each bag is reusable.
To sanitise boil in water for 1 minute or use your favourite sanitiser.