you will be able to access your computer, and do absolutely anything along as you have the permission, accessing your Minecraft is just one of the possibilities. You know how you can open command prompt and type commands that interact with your filesystem etc? You will have that access from your phone. Simply put, the command prompt of your computer will be remotely accessible from your phone.
Also will I be able to start or stop my server even when my pc is off. That's the last thing I wanna know. And thanks for all the information. Port Forwarding works on my PC so yeah
Running server means, that you have a process running that is capable of understanding and responding to Minecraft protocol. Meaning that it can interact with stuff. No, your pc is no more useful than a rock while it's not running. Just like an unconscious person can't answer, a computer can't either.