If the built-in People Tasks component RequestApprovalEmail is triggered, it sends an email with task details. This email contains a link to a frontend page where the user can see the task. To build that link, the component needs to know the frontend URL.
The default value for the PEOPLE_TASKS_DASHBOARD_URL variable is
In your installation, this is going to point to your People Tasks Dashboard implementation. You can use the Appmixer SDK to build that Dashboard or you can write your own. The backend API for that is described here.