An 'unloader' is a device - like a brace - which changes the forces going through the knee in order to protect a damaged side.
It may be useful when the joint has collapsed on one side - perhaps due to an inadequate meniscal shock absorber - and also where the joint on one side needs protecting, such as after cartilage repair procedures.
Such a brace is designed to open up the joint on the affected side as the person bears weight, thus 'unloading' the vulnerable joint cartilage on that side.
An Update on Unloading Knee Braces in the Treatment of Unicompartmental Knee Osteoarthritis from the Last 10 Years: A Literature Review.. Mistry DA, Chandratreya A, Lee PYF. Surg J (N Y). 2018 Jul 2;4(3):e110-e118.