The Keystone World is the world that is our (the faithful readers) reality. The Keystone World is home to one of the two nexuses of reality, The Rose (the other being the Dark Tower itself). Time only flows in one direction in this world. There can be no travelling backward in time to correct errors.
The Keystone World is where Stephen King writes, which is why Roland and Jake must save him in 1999 (to prevent a van hitting him and killing him before finishing The Dark Tower Series).This world is also where the Tet Corporation operates and where the majority of walk-ins end up.