Sure! NIP-42 is a way for clients to authenticate to relays. From the user perspective it’s seamless and invisible to you.

But it enables a relay to know what user/pubkey is connected to their relay. This is primarily used for paid relays (ex. filter.nostr.wine) or relays that only send your DMs to authenticated users (ex. nostr.wine).

filter.nostr.wine for example before NIP-42 required you to change the URL to include your npub so they knew who was connecting. However that isn’t very secure and it didn’t provide true authentication.

I’m sure there are a lot of use cases I’m missing. But at a high level it allows a relay to verify who a user/pubkey is that is connected to their relay.

Hopefully that helps explain it.