Nintendo has finally revealed what that mysterious 'C' button does on the Switch 2. In short, it's GameChat!
With GameChat, you'll be able to communicate with friends and family via a microphone built into the system that can pick up your voice from a distance. If you're in a noisy environment, it'll still pick up your voice easily.
It'll also work in handheld mode, and you'll be able to extend the screen of other players so you can watch and help out in real-time.
The C Button will pull up a chat menu instantly, and you can also enable a separate camera accessory. This has been confirmed to work on Super Mario Party Jamboree + Jamboree TV.
Game Chat will require Nintendo Switch Online to function, but Nintendo will be enabling a free trial period for the functionality.