It's different for everyone but like the person above mentioned it's feeling like something bad will happen or already did. For me, I feel like I'm trapped in my own mind, feeling an immense amount of fear overcome my body, I get weird bodily sensations as well. A lot of tingling in my head and body. My chest will feel super tight. When my anxiety was at it's worse I would derealize so everything around me felt fake. One time I derealized so bad I started doubting my son was even real. Lol I laugh at that now that I've learned to control a bit more but it was definitely scary to go through.