What is this on the top of bud ?

Foreskin!

what else fox tail affects ? some people says it lower the thc ?

Your plant is starting to foxtail. Fox tailing describes a condition where your buds take on a strange and irregular shape. As the name suggests, this frilly and elongated appearance looks remarkably like the tail of a fox. The main factor responsible for fox tailing revolves around stress, both above and below the soil level. Overall your bud will be fine.

Don't worry, foxtailing won't hurt your nuggets. Some people think it looks ugly, but i think it's kind of cool when nugs do it. If it's a deformity, then the same thing applies. Just an odd-shaped nugget, is all.

What does fox tail do and can i do something for it ?

Looks like a fox tail or deformity.