All the different worlds are connected. Deneki, Dragonlance, Eberron, Toril, Dominaria, Ravnica, other DMs worlds… It's all connected.

Crystal Spheres

Each world is contained within a Crystal Sphere. A literal crystal sphere. Their size varies immensely. Take Realmspace for instance, the Crystal sphere that contains the Forgotten Realms. It is 6,400 million miles in diameter and contains several bodies. Within it is the world of Toril, along with several other, inhabited planets and a star called the sun.

Phlogiston

So where are all crystal sphere's contained in? It mainly consist of a "liquid" called phlogiston. It is a very unstable and dangerous material. Countless of crystal spheres have been destroyed when their walls were breached and the phlogiston could enter.

Travel through Phlogiston

How does one travel between the Crystal Sphere's? By using a ship called a Spelljammer. A spell jammer creates a shield around the ship, blocking the phlogiston from entering. Most ships can only create such a shield for a limited time, or need to resupply on food and air while travelling.

The phlogiston flows throughout space and in some instances it flows in only a single direction. This creates rivers that a spelljammer can follow, and allows travel between Crystal Spheres. A sphere can be reached, if a a phlogiston river flows to its direction.