What is fury +20% and how does it work?

I keep seeing skill books like: +20% fury when HP is > 50%. But I’ve never seen in the game what fury is. It’s not listed among skills, glyphs and I can’t find its description, so I’m confused.

Hi,

Thank you for reaching out. Fury is the blue progress bar that’s getting filled up when the character attacks.

Thank you! Could you please explain how fury works?