A media change involves changing out the 'stuff'(media) inside your filter. Changing carbon, fine filter pads, cleaning sponges, and rinsing biological media are all examples of media change in an aquarium.
The denser the media the less airflow through it.
There are many different filter media commercially available. They range from fine fibrous wool type media through activated charcoal which both need to be replaced regularly to the various ceramic noodles and filter sponges that can be rinsed and reused many times. I prefer to use a mixture (layers) of several types of media including all the above in a power filter.
its a filter that blows air into the water you can usually get them at your local pet store
If you have a power filter it should come with the required filter media. Firstly follow the directions on how to position the filter and where to place the media. If the aquarium is a planted one there is no need to do much other than turn it on because the plants will have some good aerobic bacteria that will end up lodged in the filter media where you want it. If the tank is a bare tank you will need to cycle the filter either by adding some old but freshly used filter media or purchasing some filter cycling bacteria at a pet shop. There is another more complicated method using Ammonia but I won't go into that here. If the filter is air as opposed to power driven, you can use whatever filter media is recommended by the pet shop and use the above techniques to cycle it.
Most likely, no. The filter media (paper) inside the oil filter may not be compatible with fuel.
Your filter will tell you on the side of it how much media sand to put in it.
Social Media and Filter
to decide whether a media piece is meaningful or not
That depends on the kind of filter you have. The best filter you can get is a power filter with compartments for different filter media. With one of these you can easily clean out one media at a time thus keeping the cycled bacteria in there. The worst filter of the lot which IMO should never be used, is an undergravel filter because it can not be serviced or cleaned at all. A filter should only be cleaned when the water flow is badly reduced. Only replace some of the media and only rinse loose rubbish off what you keep. The reason for this is to preserve the good Ammonia and Nitrite removing bacteria that have formed in the filter.
There are several places one can purchase filter media for aquariums. Examples of such places include CD Aquatics, Pets At Home, Aquarium Parts and Amazon.
The proper collective name is 'filter media'.