It will be a series of games involving an almost entirely two-dimensional universe.

An example may be found in the picture aside. It's a small two-dimensional planet, thus it's a disk. Like all two-dimensional games, the inside of the planet may be viewed as a slice of a real one.

The "surface" of a two-dimensional planet being a simple line, walking-only people are severly handicaped: They may only move to left or right. If jumping is limited in the real world, it takes a whole sense here, because it allows to pass small wall, as there is no other way to go trough.