Discoid refers to a round-shape. Lateral means outside or away from the body. Meniscus (single) or menisci (plural) refers to a type of cartilage found most often in the knee.
The normal meniscus is a mature crescent-moon shape. A discoid meniscus is usually a very early crescent moon shape but thicker in depth. It's more common to find a discoid meniscus in the lateral edge of the knee joint.
The problem with this is that the discoid shape makes a barrier to normal contact between the articular surfaces of the knee. This unusual anatomy develops before birth. OCD a fairly rare problem but a discoid lateral meniscus is even more rare, affecting people of Japanese origins more than any other group.
It can result in mechanical deformities and a condition called osteochondritis dissecans or OCD. In OCD the cartilage and a thin layer of bone pulls away from the joint. It sounds like this may be what you are describing happened to you.