It's a place where you can put whatever you like, usually its good to fill it with a quality activated carbon.
See, there's three types of filtration.
Chemical (use of carbon etc to purify the water)
Biological (use of good bacteria to process and break down the waste into less harmful substances)
Mechanical (trapping particles for removal from the system)
Your undergravel filter is your biological, because thats where the bacteria live that filter the waste. It also acts as a mechanical filter to trap debris, thats why we siphon it clean sometimes.
The hang on back filter typically acts as a mechanical filter to trap particles in a pad. It also acts as a chemical filter by using carbon to adsorb impurities in the water, remove the yellow, etc. Many hang on back filters now have a wheel or something as an area for bacteria to live, to function as a biological filter as well.
So, you have the three bases covered with your setup, its a good setup.
Get a jar of activated carbon, fill the media filter cartridge, and rinse it well in tap water till it runs clear. That step is important because from the shipping process, it's very dusty and will put some black powder into your water and make it murky if you don't. People usually only make that mistake of not rinsing it once.
Over time, as you get into the hobby some more and experiment with different things, you can use the media filter with different things such as phosphate removing media or ammonia removing media, it all depends on what you'll be keeping, the bio-load, your feeding schedule, any problems with algae, etc.
If you ever notice anything strange going on with the tank, just let us know and we'll make the best suggestions for you.
Enjoy setting it up and cycling it. Start with just a few fish, wait about 6 or 8 weeks, and go from there. Biggest mistakes are too many fish at the beginning, and overfeeding as well. We can talk about the cycle process if you want the science part of it, if not, just mark the calendar and wait it out... it's tough, but will give you a good impression of the hobby instead of some bad experiences at first.
Kind regards