Dreamwalking is the ability to leave the barriers of one's own mind while asleep. It allows the dreamwalker to enter someone else's dream, and observe or control them. Dreamwalking is a form of telepathy, and therefore a psychic power. Humans can dreamwalk using African Dream Root. Angels and some demons can enter dreams at will.
Instances of Dreamwalking
Azazel comes to Sam in a dream while he is being held with the other Special Children at Cold Oak, South Dakota. He tells Sam that he wants them to fight to the death, and that the survivor will lead a demon army. He also shows Sam the night of his mother's death, and reveals that he fed Sam demon blood.
Later, Jake Talley tells Sam he had a similar dream visit.
Jeremy Frost has been using the herb African Dream Root to facilitate dreamwalking. He has learned to kill by dream manipulation. He kills his doctor, and imprisons Bobby in a nightmare about his wife. Sam and Dean obtain some of the herb from Bela so that they can enter Bobby's dreams and wake him up. They brew a tea from the root and add Bobby's hair before drinking the concoction. Dean finds Bobby and is able to convince him that he is dreaming. Sam is nearly killed by Jeremy, but is saved when Bobby wakes up and the dream ends.
Jeremy's next target is Dean. Dean stays awake as long as possible, but when he can no longer resist, Sam takes takes the herb to follow him into his dreams. While Sam confronts Jeremy who overpowers him with his control over the dream, Dean faces down a manifestation of his own self-loathing and kills it, symbolicaly killing that part of himself. However, he is unable to wake up and his dream self morphs into a demon version of him, a manifestation of his fear over becoming a demon and it taunts him that he can't escape Hell and this is what he'll become. Meanwhile, Sam is nearly killed by Jeremy, but is able to use the dream root he has taken and possibly his psychic abilities to bring Jeremy's father into the dream and free himself. While Jeremy is frozen in fear, Sam hits him twice in the head with a baseball bat, killing him in the dream and in real life and waking himself and Dean.
As Uriel cannot find Dean, he contacts him via his dreams and forces Dean to reveal his location.
In a dream, Dean is fishing on a dock. Castiel appears beside him, and gives him an address where they will meet. He refuses to speak in Dean's dream, fearing he will be overheard.
Sam seems to be having dreams about Jess, who tells him that he cannot run from his fate. It is actually Lucifer, who reveals himself in the last dream. He says he cannot find Sam, and Sam does not reveal his location. Lucifer tells Sam that he is grateful to be free, and that Sam is his vessel.
Anna comes to Dean during a dream set in a strip club. She explains that she has been Heaven's prisoner, but has escaped. She cannot find him because of the Enochian Sigils on his ribs, and asks him to meet her later.
Zachariah comes to Adam in a dream about his childhood playground. Castiel hid Adam with sigils, and the angels cannot locate him. Zachariah convinces him that the Winchesters are wrong, and that he should give up his location.
After Charlie Bradbury is poisoned by Djinn Jennifer O'Brien and the antidote doesn't work, Sam and Dean figure that she is stuck in a loop replaying something over and over and as it is similar to a dream, they can use African Dream Root to enter her mind and break the loop and free her. Dean enters Charlie's mind and finds her trapped in the game she stole, The Red Scare where she endlessly must defend her comatose mother from super-soldier vampires. Dean realizes that Charlie's greatest fear is losing her mother and if she stops trying to fight back, they can break the loop and break free. Charlie sadly agrees and the loop breaks and Dean and Charlie wake up.