The filter is the life support part of the aquarium, without one your Discus will die.
The primary function of the filter is "biological" to deal with the ammonia the fish produce, then the nitrite and this in turn becomes nitrate. As water will flow through the filter, it also remove any sediment from the water, it is this that blocks filters and is removed by regular cleaning. A third filtration stage is "chemical" this is where you can add a media or resin to the filter to polish the water, or remove phosphates, nitrates and all manor of other dissolved substances.
A simple air driven sponge will clean the water biologically, also mechanically. Internal electric powered filters do the same. External canister filters and sumps have the additional function of chemical filtration as an option.
Filter bacteria require well oxygenated water to thrive and work, so always get a filter bigger than you think you need. They can all be turned down, but none will work above their maximum capacity.
Filtration in my experience is best kept simple. A guide to how to mature and maintain the filter bacteria can be found here..