Someone who's dispossessed has had something important, like their home or their sense of safety and security, taken away from them.
Most people who are described as dispossessed have lost their possessions, the things that belonged to them. This usually includes essential things including a place to live, a homeland, or basic rights. The word possess, or "own," is at the heart of this adjective, from a Latin root meaning "to have and hold," or "to control." Taking away someone's property, or their ability to control their own life, leaves them dispossessed.