The knee joint has two horseshoe or C-shaped pieces of hard cartilage called the menisci. There's a medial (inside edge) and lateral (outside edge) meniscus. Both menisci are attached to the bone.
The medial meniscus is also attached to the joint capsule and one of the ligaments. These extra attachments make it less mobile but also more likely to tear under force. In fact the medial meniscus is torn twice as often as the lateral.
When the meniscus is torn and the tear goes the length of the cartilage it's called a bucket handle tear. If you could touch the torn meniscus, it's possible to lift the torn section up like a bucket handle.