Answer:

A Rental Key is a code that may be used to activate Foxhound Version 2 as a Rental Edition copy after

• installing a fresh copy of Version 2 without upgrading from an existing copy of Version 1.x, or

• upgrading an existing copy of the Version 1.x Evaluation Edition to Version 2.

In both cases the Foxhound Activation page will appear automatically when you start Foxhound, and entering a Rental Key will be one of the alternatives.

A Rental Key may also be used to extend or renew the rental period for an existing installation of the Rental Edition for an additional or subsequent 30-day period. A different Rental Key is required for each 30-day period; see the Renew or Upgrade Foxhound button on the About page.

You can purchase a Rental Key here:

See also...
What are the differences among the different editions of Foxhound: Rental, Basic and Extended?
What are the differences among all the activation keys?