What does it mean if you dream ghosts are holding you down in your dream and you cant shout or move?

This type of dream is associated with the common phenomenon of "sleep paralysis." It occurs when one partially awakens during sleep and senses the paralysis of the muscles and speech that is normal during sleep. It's as if our mind is awake or half awake while our body is motionless in sleep. This state is often accompanied by a feeling of someone (imaginary) being in the room, or a demon sitting on your chest, or a ghost as you've described. It can be classed as an anxiety dream or nightmare. Psychologists don't usually consider this type of dream to be meaningful material for interpretation; it's more like the result of a malfunction of the sleep cycle.