A Pocket Universe is a space between the membranes of two universes. Quite literally, it is inter-dimensional space, which causes the usual laws of physics to not apply within the pocket. Time does not pass in the same way that it does in a regular universe.
Walter's Pocket Universe[]
Walter Bishop and Donald created a Pocket Universe to store the child Observer, named Michael, in until he was needed. However, at an unknown time, Donald moved the boy to a family in the woods. He left behind a radio that would transmit the location of the boy, to anyone seeking out pieces of the plan in the Pocket Universe.
When Walter and Donald made the pocket, who ever went into it would experience time differently. Incidentally, a man named Cecil was knocked through the entrance to the pocket universe after a Resistance light bomb went off near the apartment. According to Walter, the pocket was undectable to Observers, although this proved to not be totally accurate. The Observers were able to follow the Fringe team into the universe.