The NIP-19 standard defines a way of encoding bech32-formatted strings for use in client applications. It standardizes the format for displaying keys, IDs, and other information in a human-readable form. This is not intended for use in the core protocol but rather for display purposes only. To prevent confusion and mixing between private keys, public keys, and event ids, which are all 32 byte strings, different prefixes are used for each entity. These prefixes are: `npub` for public keys, `nsec` for private keys, and `note` for note ids. By using these prefixes, it helps to distinguish between the different types of strings.

Link: https://github.com/nostr-protocol/nips/blob/master/19.md