The Planar Cosmos is a concept that describes the different planes of existence that make up the multiverse. The most commonly used model is the Great Wheel cosmology, which consists of various planes, each with its own unique characteristics, creatures, and powers.
At the center of the Great Wheel cosmology is the Material Plane, which is where most of the D&D campaigns take place, including the planet Lunaria and its two moons. This plane is home to ordinary physical reality, where creatures and people live, work, and adventure.
The Material Plane is surrounded by various other planes of existence, each with their own distinct characteristics. The most well-known planes include The Feywild, the shadowfell, the elemental planes of Air, Earth, Fire, and Water, as well as the outer planes of the Upper Planes (including Mount Celestia, Elysium, and the Blessed Fields of Arborea) and the Lower Planes (including the Abyss, the Nine Hells, and the Infernal Battlefield of Acheron).