From the presentation of Nstart:

"This is really cool stuff made by fiatjaf, that uses FROST to split your nsec in 3 (or more) and distribute each shard to an independent trusted remote signer. This will give you a bunker code that you can use to log in to many web, mobile and desktop apps without exposing your nsec. If you ever lose your bunker code, if the signers vanish from Earth, and it stops working, or if it gets stolen by a malware virus, you can use your nsec to create a new one and invalidate the old one."