Realms

The Realms are the invisible dimensions in which all magic comes from. The Guardians are in place to protect these Realms and the Worlds they build are located at the metaphorical "edge". It is believed that the Realms are not places of physical being, but pure magical energy and, if it were possible to travel to one, they would be inhospitable to the point of fatal to any mortal that crosses the threshold.

Gateways
The entrance and exit to these Realms are known as Gateways. There is little know about exactly what these Gateways are and how to open or close them, but it is known that they are not physical places, but rather exist purely through magic.

There are, however, physical Gateways that connect the many Worlds together to the Nexus, but these were not made by the Children, nor by the ancient humans. Some think that they were made by the Guardians or that they appear naturally as the Worlds connected.