A brace for the knee is a constraint of knee movement, usually made of fabric and plastic and/or metal.
A functional brace to allow protected knee movement.
There is a lot of controversy about braces - caused by a mixture of unreasonable expectations from users, fierce competition amongst manufacturers, and a plethora of poorly-controlled and relatively unscientific studies. There is also a lot of ignorance amongst both the medical profession and suppliers themselves.
But there are some good scientific studies and some excellent manufacturers and suppliers. Some braces are enormously expensive, and it is important to understand that there are considerable variations in brace design and manufacture, depending on what the manufacturer is trying to achieve.