It’s a Continuous Passive Motion machine, which will do cyclic bending and straightening of you knee to the degrees that you set. You can gradually increase the degrees of bend and straightening over time as your tissues stretch. This is a good way to increase ROM when it’s performed on a daily basis, but if you’ve had arthrofibrosis long-term you may need to continue doing CPM because you will likely lose ROM when you stop.
Anyone looking to buy one should look for one that has a hand-held digital controller, and should always stay within the pain-free zone. There will be some discomfit at the limits of ROM but you don’t want to tear tissues.
There is more info on how to use a CPM under “Key Points”, and a photo of one. Some hospitals in the US sell recon CPMs on Ebay.