Reactors in their most simple form hold filtration media and have water forced through the media. Bulk Reef Supply has a couple of models, and the Phosban 150 model by Two Little Fishies is also very popular. (Bulk Reef Supply has some good videos on their site as well as YouTube you can watch to get a better idea.) They generally use a dedicated powerhead to pump water through them from the sump and return the water to the sump, but many can be used as HOB units as well.
Just for purposes of clarification, what willbratz is advocating is using the sump as what I would consider to be a simple refugium with live rock and the chaetomorpha algae. The chaeto is both a breeding ground for pods as well as working to remove nitrates. The live rock will do the same.
Here is a video I made of my sump. I do not have a refugium section like willbratz, but I use a small amount of bioballs, an oversized skimmer, filter socks, and a media reactor. (It's the BRS media reactor, but you can't see it very well due to it being under the stand.) My method of nutrient removal I guess would be more chemical and mechanical where willbratz method is more biological. Either method can work, but the refugium method is far better for raising pods.
Aquarium sump explanation - YouTube