The Stifle, in a horse, is fairly similar to a human's knee. The biggest difference is that a knee is designed to be straight, when weight bearing, as opposed to the stifle joint, which is intended to be bent when weight bearing.
The Stifle joint is susceptible to many different causes of injury, as well as several diseases. These can all lead to lameness in a horse.
Arthritis, degenerative muscular-skeletal disease, genetic abnormalities, and infection can all lead to temporary or permanent Stifle problems.
It is always best to have your horse seen by a veterinarian when a Stifle problem is suspected.