Normal space, which the Creator refers to as 'Spacetime Four', has only the three dimensions of space, and one dimension of time (i.e. time only passes forward). This is scientifically known as the normal space-time continuum; an object's location within spacetime can be described as its location in three-dimensional space at a certain point in time.
The Creator describes Spacetime Six as the place where the three dimensions of space join the three dimensions of time, so time can also go other ways there, i.e. not only forward but backwards as well. When in Spacetime Six, Boston mentions that he can see every past and future, and wonders which one is real. This suggests that not only can he see the past and present of his own timeline, but also parallel dimensions as well, i.e. alternate timelines. So it is truely a place where one can be at any place at any desired time, but only as a consciousness, an observer, unable to interact with any tangible world.