The garden is supposed in Gen 2:8 to be “eastward in Eden”; what does that mean?

As “eastward” is relative to something, and since the book is held to be written by Moses, presumably Eden is east of the Promised Land, although this is not altogether clear. “East” of the Promised Land would include, consistently with other uses of the word in the Bible, due north and northeast, along the Fertile Crescent; invasions “from the east” were actually, at least in their last hundred miles, from due north. As to where Eden was, the answer, as commentators say, can only be inferred from the proffered details. Mostly, we must look at the rivers, on which, see below.