A free use work ("Free-to-Use") could be considered any work which is publicly made available for use yet the creator or copyright holder has reserved all or some rights (All/Some Rights Reserved). Rights are retained by the holder yet the work is made free to the public.
There is not a precise way to describe "Free-to-Use". In some sense, it is very similar to releasing a work under a Creative Commons license or using a work through Fair Use, but without the release to copy the work. Typically "Free-to-Use" works often appear as digital objects or content which can be linked to or embedded for use (think of services like YouTube). The copyright holder has granted a content delivery service or has released content through another public method which allows the general public to access and use the content by linking to the work, or embedding the content.
Making a copy of the work is still illegal under Copyright but use of the work through mechanisms provided by the service is generally okay.