Use it for questions related to the Token module, or a similar form of token replacement system. Don't use it for authentication tokens.
Tokens generally refer to the tokens implemented by the Token module, but it could also refer to any over token replacement system implemented by other modules, such as the %-placeholders used in Drupal 6 to replace dynamic information reported in the email messages sent from Drupal to the users in specific occasions (e.g. when the users forgot their password).
In Drupal 7, the part of the Token module has been merged into the Drupal core module; modules running in Drupal 7 or higher can use the token replacement system for their strings, and define new tokens that are used from any module that use the token replacement system.
The only part that has not been merged with the Drupal core is the user interface, which in the Token module includes the field set listing all the implemented tokens.