Plicas are remains of fetal development. At an early stage, the knee was divided in to three compartments. Later, the three compartments form the synovial capsule. In the adult the remains of the Plica form sleeves of tissue.
Plica are harmless and of no consequences unless they become inflamed as part of the capsule. Chronic inflammation makes the plica thicken and prone to more irritation.
MRI is the best method of visualization and treatment of the inflammation is anti-inflammatory medication in most cases.