Research the nitrogen cycle, understand this will help in maintaining your aquarium.
In a nutshell, waste from fish, uneaten food and dead plants breaks down into ammonia. There are colonies of bacteria that convert the ammonia into nitrites and the nitrites into nitrates.
Biological filteration is that which has media designed to house these colonies of bacteria. The filters will remove the ammonia and nitrites from the water. You will then have to keep the nitrate levels low by doing regular partical water changes.
The mechanicla media will trap the debries in your aquarium. This will give your aquarium water a cleaner look. If you frequently clean the mechanical media and remove the fish waste, uneaten food or dead plant material, you will lighten the biological load. Lightening the biological load will have and end result of lower nitrates, but water changes will still be necessary to keep the nitrate level acceptable.