Well....it's which slice, or "page" of the dimension of "time", (as a single, whole object, from beginning to end) you are experiencing at this particular moment in your consciousness.

I don't really lean much towards the story's in Eschaton's book, but if you can conceive of time as the fourth dimension, like a long winding snake, from the beginning of the universe - to the end of the very last star blinking out of existence.

Now slice "pages", or "cards" into that snake, these slices are our 3-dimensional perception of individual moments in time as a whole.

Think of time as a gigantic flip-book, as the moments of existence pass by, your consciousness jumps to perceiving the next page of the flip-book & time is set in motion.

Here is a cool video that explains this concept a bit better than I can. 

Plus the guy uses video-feedback to kind of show an example of the "time-snake" and the "pages of the flip-book" concept, allowing you to watch more than one page go by & then continue to exist even though we are not perceiving it anymore as...NOW...in the 3rd dimension.

This video is one in a series of them about multi-dimensional theory. 

It is one way (of many) to explain how we in the 3rd dimension can barely conceive of a 4th dimension (time as a whole, not broken into moments), because it is the "shell" that  shapes & creates our 3rd dimension! 

Same exact way our 3rd dimension shapes & effects the 2nd dimension & a conscious 2-D being would have no concept of there even being a 3rd dimension, as it is the "shell" that shapes & contains his 2-D world.  So he would have to somehow step outside of his 2-dimensions to experience & actually understand that 3-dimensions are even possible. 

Although, he could think about more dimensions existing & prove it by testing how the 2-dimensions of his world are effected by an unexplainable 3rd!!

Here is a link to video number 1-of-2, called Imagining the Tenth Dimension.

Pretty cool stuff!!

WS