Media can be lots of things depending on the make of your filter, basically it is something porous that can house bacteria, such as filter floss, ceramics, sponges and similar.
One of my filters has two slots, one for carbon and one for a sponge, but as carbon isn't needed I removed it and put in two sponges instead. My other filter (in a different tank) has 3 compartments, two that hold sponges (or could house carbon) and one in the middle that is like a cage full of something that looks like chalk (I think it is like ceramic is some such thing)... my sponge bits are quite spacious so I've jammed a third sponge in there to gather bacteria in the case that I should have a problem in my betta tank... so in the future if the bacteria dies off I can just move a sponge straight over.