DE stands for diatomaceous earth, skeletons of tiny critters that forms a super-fine powder like powdered sugar. You buy it at a pool store or the pool section of WalMart, K-Mart possibly Costco. Here's where my knowledge is shaky: I believe you make a slurry of water and DE and add it through the skimmer (the skimmer part is right).

But with a DE filter you have to add DE after you back wash. I don't know how much: That should be available by looking up the manufacturer and model number. I don't know how you work it if you don't know how much to add when you recharge it.