The word 'syndrome' means a group of clinical findings that tend to be associated with one another, and likely to have a common cause. The plica syndrome is a group of clinical findings in the knee which appear to be related to damage to a rather enigmatic structure called the synovial plica.
The inner walls of the knee joint are lined with a special tissue called the synovium, the cells of which secrete the fluid that lubricates the joint. In many people, but not all, a thin sheet of synovium may stretch from the wall across to the more central structures and this would be called a synovial plica.
Mostly the plicae are thin and stretchy, and just move with the other tissues of the knee when the knee bends and straightens. Sometimes, however, they get traumatised and swell, and catch on the knee structures, or between structures and then then can be the cause of a lot of misery, such as local tenderness, snapping, clicking and the knee may catch or give way.
Here are several sites that go into more explanation -