Demiplanes are extradimensional spaces that come into being by a variety of means and boast their own physical laws. Some are created by spells. Others are natural folds of reality pinched off from the rest of the multiverse. Theoretically, a Plane Shift spell can carry travelers to a demiplane, but the proper frequency required for the tuning fork can be extremely hard to acquire. The Gate spell is more reliable, assuming the caster knows of the demiplane.
A demiplane can be as small as a single chamber or large enough to contain an entire realm. For example, a Mordenkainen’s Magnificent Mansion spell creates a demiplane consisting of a foyer with multiple adjoining rooms, while the land of Barovia exists entirely within a demiplane under the sway of its vampire lord, Strahd von Zarovich. When a demiplane is connected to the Material Plane or some other plane, entering the demiplane can be as simple as moving through a portal or passing through a wall of mist.
Demiplanes are limited realities shaped according to the will of whoever created them. Adventures in demiplanes might let characters explore how they would shape reality to suit their desires and ideals, or confront distortions of reality crafted by villains.