Arthritis means, simply, joint inflammation and it can have many causes from autoimmune disease to infection to trauma. Weight often causes or worsens arthritis by putting added pressure upon weight-bearing joints like the knees and hips, bringing opposed bone surfaces into close proximity, grinding them against each other with greater force and eroding and inflaming joint cartilage. Furthermore, obesity itself is a condition associated with generalized inflammation. 

Often the damage to joints caused by arthritis will leave permanent scarring and at least limited pain, but weight loss will usually slow or stop the progression of damage and by reducing joint pressure, often dramatically reduce pain.