What is it? Good question. Threadless puts up a shirt - exactly one shirt - for $10. It could be a guy's shirt, a girl's shirt, it could even be an exclusive select tee that is normally $25. It stays there as long as it takes for someone to buy it.
When it gets sold, another shirt takes its place. It goes on and on ad infinitum. I just bought a $25 for $10.
You're welcome, I guess. Or sorry.