In a nutshell, remix is a thing where you get assigned an author and get to take one of their stories that you pick and change it up however you want. Expand it, change the POV, change the setting, whatever.
Specifically, musesfool asked me to try and get some more JE people interested, because right now only one person is signed up in JE, and they can't remix themselves, obvs. The one limitation is that you have to have at least some fic that's KAT-TUN or NEWS or Arashi, because the system only allows for fandom matching, not band-specific matching.
However, once you have an author assigned to you, you can remix anything of theirs you want. For instance, if you got matched with me because you said you could write Arashi, you could remix any of my Kis-My-Ft2 fic, or even do something from one of the 8 million other fandom's i'm in. The qualifying fandom rule is only so that people have a better chance of getting something they're willing to write. Last year, for instance, I got assigned Arashi but then ended up writing Butoukan as well, and it all turned out fine.
If it sounds interesting, even if you're a little concerned about what kind of fic/pairings the person you get writes, you should go for it. I'll tell you that I usually end up writing something I've never written before and it's a good thing for experimentation, and sometimes I've even written in fandoms I've never written in before. If you're multi-fannish at all, this is a good time, I promise. And even if you aren't, anything's worth a try, right?
The FAQ and directions for Remix Redux are right here, and also in there is the link for how to get a Archive of Our Own code if you don't have one. Remix is run on AO3, not LJ, but don't let that stop you. It runs exactly the same as it did when it was on LJ, and before that on its own website.
Aside from JE, other fandoms that could use a few more people are Avatar the Last Airbender, Hawaii 5-0 (2010), Leverage, and White Collar.