Lacking a better term, that is a reverse fray. The inside of your jeans (weft) got stressed enough to break apart and that's how it looks on the outside. It looks like you double cuff and this is not uncommon for the denim to to react this way. I would not worry about it but that's me. It will get bigger over time though, then you may want to sew it up.
2/4/13 at 2:01pm