卧床不起 is a fixed four-character phrase meaning 'to be bedridden' — literally 'lie in bed and cannot get up'. It is used to describe someone who is too ill, injured, or weak to leave their bed. The structure is 卧床 (lie in bed) + 不起 (cannot rise), forming a resultative verbal phrase. You can place it after a subject, often with 了 to indicate a completed state: 他卧床不起了 (He became bedridden).