Biological filtration is the use of bacteria to convert toxic ammonia into nitrite (NO2 – also toxic) then into the less harmful form of nitrate (NO3). The nitrates are removed with partial water changes to keep them at a reasonable level.  The establishment and maintenance of an effective nitrogen cycle is the most important essential for successful fish keeping.  It is explained in more detail here.

Once your tank is up and running, and your filter is fully cycled, biological filtration takes care of itself and looks after the quality of your water with very little intervention (except for regular water changes to remove excess nitrates and replace lost minerals from your water).  Never wash your filter media under the tap (as chlorine and other ‘additives’ from tap water will kill the useful bacteria that lives there).  Instead, rinse it through, when that becomes necessary, in some of the tank water you have removed in the water change.  When your filter sponges become clogged after a ‘clean’ so that they do not work correctly, they need replacing.  However, only replace a ‘portion’ at a time as you need to keep the beneficial ‘Bacteria’ to do their job.