Which tool or mechanism defines what resource a user may access and what he or she may do with those resources?

Answer: Authorization

Explanation: Authorization is the tool or process that is used as the privilege/permission to user to carry out the process. This process is required to maintain the security and so that the resources of the system can be used by the authorized user.

It is implemented on the resources like files, data, programs etc. Authentication process is carried out by the verification of identity of the particular user.