it's your nostr secret key, nsec if you are using amethyst either you put it into amethyst or you put it into amber if you don't have amber, get amber i kinda had problems with amber though, and amethyst, with it popping up all the time and i couldn't figure out how to fix it so, yeah, my guess is you have an nsec in amethyst and never put it anywhere else, but it is at risk of being lost if you don't put it somewhere else...

nsec is your secret key. You must have input it to login to Amethyst It's not specific to webapps or any platform. You can use it to login to any Nostr app Best to avoid using your nsec anywhere and use keystore apps like nsec.app or Amber

I just saved my nsec and npub in my password manager as a note