The Client Interface is for providing general information on a Client.
This is only available for clients, these won’t work on a sever!

Where to find the client class?

The Client class is a globally registered Symbol, so you won’t need to import anything, you can just use the keyword client

What to do with it?

client.player returns the player running the client as an IPlayer Object.
client.language returns the language the client runs on as string.