Citra web tokens are unique identifiers that allow your personal Citra installation to be able to log in to Citra's Web Services, without having to save your email and password. Logging in to your Web Services account allows you to provide more detailed statistics with telemetry, among other things.
In the same browser, click here to retrieve your token.
Your token is displayed in the text box, copy it into your clipboard.
If you'd like to generate a new token, click the refresh button beside the text box. Note that this will invalidate your previous token and thus sign you out from within Citra.
Installing A Token
Open Citra, and go into the Emulation → Configure... menu.