The Inner Worlds was the name of a group of planets within the Federation, as distinct from the Outer Worlds.
Little is known for certain about the Inner Worlds as they were referred to less frequently than the Outer Worlds.
Chesku spoke the possibility of Earth being reunited with the Inner Planets, but it is not clear whether the terms are interchangable or indeed what he was actually referring to.
The President of the Federation's styling included the phrase Lord of the Inner and Outer Worlds.