"Arth" means Joint "itis" means inflammation. Therefore it means inflammation of the joint. This inflammation is the reason of swelling & pain in the joint and disability in walking or using that particular limb.

Let us understand that if our joint is severely damaged by arthritis or injury, it may be hard for us to perform simple activities such as walking or climbing stairs.One may even begin to feel the pain while sitting or lying down.

How the Normal Joint Works
Let us start with example of the knee which is the largest joint in the body. Nearly normal knee function is needed to perform routine everyday activities.

The knee is made up of the lower end of the thigh bone(femur),which rotates over the upper end of the shin bone (tibia), and the knee cap(patella), which slides on the groove on the end of the femur. Large ligaments attached to the femur and tibia provide stability to the knee. The long thigh muscles give the knee strength.

The joint surfaces where these three bones touch are covered with articular cartilage, a smooth substance that cushions the bones and enables them to move easily. Two loosely attached soft tissues (Menisci) also provide cushioning effect.
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All remaining surfaces of the knee are covered by a thin, smooth tissue liner called the synovial membrane. This membrane releases a special fluid that lubricates the knee which reduces friction to nearly zero in a healthy knee.

Normally all these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain muscle weakness, and less function.