These large foam block pre-filters connect to the inlet of your pond pump and often will replace the tiny pre-filter sponge that comes standard with some pond pumps. These larger filters take much longer to block up and will extend the life if your pump significantly, compared to a small standard pre-filter) if you choose to install one. They are a handy budget option for small ponds however they are much less effective than a pressure, gravity or all-in-one filter.
Need help choosing a suitable filter for your pond? Click on one in your price range - you will go through to a page with a lot more info which will help with your choice.