Psychosomatic means mind (psyche) and body (soma). A psychosomatic disorder is a disease which involves both mind and body. Some physical diseases are thought to be particularly prone to be made worse by mental factors such as stress and anxiety. Your current mental state can affect how bad a physical disease is at any given time.
There is a mental aspect to every physical disease. How we react to disease and how we cope with disease vary greatly from person to person. For example, the rash of psoriasis may not bother some people very much. However, the rash covering the same parts of the body in someone else may make them feel depressed and more ill.
There can be physical effects from mental illness. For example, with some mental illnesses you may not eat, or take care of yourself, very well which can cause physical problems.