Rosin filters are special, high temperature-resistant nylon bags with a mesh (various mesh sizes to choose from). You fill the filter with herbs, kief (pollen) or hashish and you put the filter into a press designed for rosin production. If you make rosin from kief or hashish, filters are absolutely indispensible if you want to make sure that the final product is free from plant material. When you make rosin from herbs, filters are optional but remember that filters protect rosin from mixing with plant particles. The finest filters (30u) are made for kief, the medium ones (50u) are great for hashish and the ones with 120 u mesh can be used with dried herbs.