A non-dissipative medium is a medium within which a quasistatic process is reversible. That is: inside it a process conducted infinitely slowly dissipates no work, creates no entropy.

For example:
Moving an object from A to B within an ideal gas - a non- dissipative medium. As the object moves it bats the gas particles in front of it forward. These bang into other particles; setting all into violent motion. (The temperature rises immediately in front of the moving object.) The same process, acting in reverse, slows down the particles behind the body. (The temperature cools immediately behind the object.) Thus the body gets struck more violently and more often on its front face than upon its back. This would bring it to a halt were it not supplied with work - pushed - to enable it to continue its journey. The more slowly it is pushed from A to B the less work is needed; dissipated into increasing the speed of the average particle in the sea of ideal gas particles. In the limit, when the body is moved infinitely slowly, quasistatically, no work is dissipated - no entropy created.

For example:
Sliding an object, from A to B, along a wooden plank - not a non-dissipative medium. Here the force resisting the motion does not necessarily get less if the body is moved more slowly. It does not tail away to zero in the limit of infinite slow motion. Indeed, as it happens, static friction is greater than dynamic friction. It would always dissipate some work to get from A to B.