The principle behind mechanical filtration is simple.  The idea is to physically remove particles, and waste matter from the aquarium water.  Many times a fine medium such as cotton, dacron, and other synthetic materials are used to trap detritus, dead plant matter, and other debris. 
Mechanical filters need to be removed, cleaned, and replaced on a regular basis.  Otherwise, waste products will accumulate, and decompose, continuing to lower the livability of the aquarium environment.  This puts major stress on the fish and other inhabitants of the aquarium.  One of the secrets to successful aquarium care in all applications is to maintain the mechanical filters.  Replacing this media will help ensure the long term success of the tank.