What does it mean when you dream that a spirit is on top of you and sufficating you?

My mom gets these dreams, but she says that its not a dream and that its an actual spirit that is going on top of her at night. She says that she tries to talk and scream but she cant. I think its a dream but im not sure. Then she says that when its about to sufficate her, it gets off and goes floating right thru the wall towards my room and she just freaks me out. Has anyone ever heard of this or knows what it means?? Please help!!