what the purpose of meniscus in joint

The Meniscus (also known as cartilage) is a fibrocartilage in the knee that has a very unique shape, design and purpose.  The purpose of the meniscus is act as a shock absorber and space filler between the bones of the knee.  The menisci are of a “C” shape which corresponds and is contoured to the surface of the femur.  They also have a wedge-like shape to them that helps with stability.  This would be similar to the wedges used as door stops or a slice of an apple.
 
As stated, the meniscus is the primary shock absorber in the knee.  It helps to dissipate the forces in the knee caused by all activities such as sitting, walking, running, jumping, cutting, etc.  The goal is to disperse the forces over a greater area as opposed to isolating them.

As in, the meniscus in the knee?

The medial meniscus is a half-circle piece of cartilage in the knee that’s located on the inner side. The purpose of the medial meniscus is to reduce the force between the tibia and femur bones during activities such as walking, running or jumping. The meniscus on the medial side of the knee can be damaged when by twisting. Cartilage tears can usually be solved through conservative treatment although surgical intervention is sometimes necessary.