The knee has two menisci, one on the medial side and one on the lateral side. The meniscus is a C-shaped piece of cartilage that are on top of the tibia or shin bone. They help to protect and provide cushion for the knee joint during activities like walking and running. A meniscus can be torn during forceful twisting of the knee joint or excessive knee bending. This can cause knee pain, popping, swelling and a giving out feeling. 

Source: medicinenet.com