Just what it sounds like. It's a sort of sleeve-type accessory that goes on your forearm, with a hole where your thumb goes. Supposedly used by "emo" or "goth" or "scene" people. They keep your arms all warm and fuzzy. They're nice for when you don't want to wear a sweatshirt, but it's too cool out to just have bare arms. (just saying, they're also good for those of us with scars on our arms..) Personally, I can't go a day without them.
They're also seriously easy to make. If you have any knee-high socks (new or old), you can just cut them and cut a hole for your thumb (sewing that area is best so it doesn't wear out as fast), and BAM! armwarmers.
"Sweet armwarmers. Where'd you get them?"
Some kid: "Why do you wear those all the time?"
Me: "To hide the scars, retard."
Some kid: "Wow. Freak. You're a cutter?"
Me: "Fuck off."