The Hours Approval tool is used to approve (and possibly edit) or reject volunteer self-entered hours. Note that hours created as a result of checking in and out of assignments or trainings never require approval, this is only for hours entered by the volunteer after the fact.
This tool's behavior can be set using its tool profile to limit the Jobs, Locations and Groups; Hide or show the Distance field; Allow editing of start dates or jobs; default to approval even if there are conflicts; Default the Supervisor to the current user; Show or hide the Send a letter link.