Sam has psychic premonitions, in the form of nightmares (while he's asleep) and visions (while he's awake). He sees things exactly as they're going to happen, usually from an outsider point-of-view, like he's watching it from a point of non-involvement. The general timeline is that nightmares are things that will happen within a few days, and visions are things that will happen within a few hours/that day at the most.