A joint is the connection between two bones. Joints and the surrounding structures allow you to bend your elbows and knees, wiggle your hips, bend your back, turn your head, and wave your fingers.
Cartilages cushion the joints so that bones do not rub together. But (1) increasing age, (2) some injury, (3) sitting the wrong way, and (4) carrying too much weight can wear and tear the cartilages. This can lead to joints damage and arthritis. Hence, joint care is necessary.