Keystone Earth is a world within the infinite number of parallel universes in the multiverse of the Dark Tower. It is the "real world" in which readers of Stephen King's books live. Death is permanent in this world and time flows in in a forward motion. It is represented by a rose that grows in a New York City vacant lot.
There is a good argument to be made that Keystone Earth is not the reality you or I inhabit. Keystone Earth is still a reality in which a Book Version of King was Written. The implication that Sai King himself was actually physically saved by Roland and Jake in our actual reality is pretty far fetched. So rest assured we are in fact not in Keystone Earth nor is there a Jake Chambers or Eddie Dean or Rose in a vacant lot or Tet building here in our reality.