An nsec is a Bech32-encoded private key. The Bech32 envelope serves exactly one purpose: visually distinguishing a secret value from a public one at a glance. The underlying key is the same 32-byte secp256k1 scalar that Bitcoin uses.
Two rules for nsec hygiene: never paste it into a website, never store it in a plaintext file synced to the cloud. Every Nostr client ships with a way to generate, import, and export one, and every client has produced at least one bug that ended up leaking them, so treat the value with the same paranoia you would a seed phrase.