Imagine your house is your mind, and outside is reality. Usually outside is sorta part of your home, your lawn being what's immediately there (like what you're watching on TV or the people you're talking to), and the stuff beyond further away.
Now a few rowdy kids start playing with fireworks, messing around with an abandoned car. Not terrible but still has you uneasy. Then they start severely damaging the car because they can. You start closing doors and windows because you just don't want to deal with it. Hey at least it's safe inside right?
Eventually they set fire to this car, are launching fireworks and might even have guns, and you just feel unsafe in your own home -- your comfort zone. One of two things usually happens here. One is you parked your car on the street too. And yes the world outside is shitty, but you can't help but keep tabs on it making sure your car isn't damaged. These are called "intrusive thoughts", basically the "what if???" thoughts that you cannot control and are both unwelcome and uncomfortable.
Or if you didn't leave your car out there, you can go to a safer part of your house. (For example your bomb shelter, just if it were actually fully empty). This is a very isolated area and you stay there until further notice. Now you've locked away most of your house (which remember in the analogy is your mind, the thoughts you have etc.) and have a small fraction left. But there's nothing to do there, it's boring. And you're trapped there.
It might not sound bad but remember, everything is still going on. Your thoughts are still there. As is reality. You just have no focus in that and can only focus on the most minimal of stuff -- like "being on autopilot".
A not terrible way to understand that emotionally is to watch a movie so awful you want to fall asleep during it. You just can't get into it. But it's at the theaters, you have nowhere to go. It's a movie designed to being out emotions, a drama that tugs at your heart strings, and you just feel nothing for it. That movie is reality, and the theater is you. The world is actively happening and you're just "zoned out", you can't focus and that's not your fault.
Being trapped inside my own mind but out of my own thoughts (don't know the term for this yet) takes it a step further. The movie is now your thoughts, and the theater is just you in general. You have thoughts, but there's no conviction to them. There's no emotional attachment. They just sorta happen, and you're an indifferent bystander to it all.
- Julietta