It's a SMS4, a USB dongle with which you can slap any DS game in to rip the game on your PC so you can play with your own ROMs.
See this like a Retrode (SNES/GEN carts) but for DS games, I can back up all my games and even the saves files!
It took me a while to finally find a decent, quick way to rip my library. I tried the Acekart 2i trick with internet, but my DS isn't able to get my wireless now for some reason. After this little investment I took a little break then I tried to look for something else a couple of months later, then I found the SMS4.
It's pretty much exactly what I wanted, you install the little ripping software on your PC, slap the USB dongle in, with the DS cart in it and BOOM you rip it and depending on the game's size it'll take 5 - 15 minutes until it's done.
Thing is, it wasn't easy to find a seller that wasn't some weird, chinese website that was asking for credit cards without accepting Paypal account, which sounded way too risky for me.
Then I found this website, which doesn't actually show they use paypal on the side, but it's actually an option when you're checking out!
I was still on the fence a bit, but I took a chance, turns out I got it and everything worked out fine!