Pool water filtration is the first step in treating your pool.
Whatever filter you choose, the operating principle of a filtration system is similar: the pool pump drains impurities into the filtration system so as to remove dirt from the water before returning the later into the pool though the outlet nozzles.
The filter will thus naturally become dirty during its operation.
Some filters are equipped with a manometer which indicates their internal pressure. If this manometer indicates an unusually high pressure: the filter is likely dirty.