It may mean that your essay starts with one geographic or specific place and moves logically to another place -- and then to another.
For instance, if you are comparing the French Revolution to the American Revolution, you'd include all of the information about France in one section and all of the information about America in the other section.
This is what the UNLV Writing Center says (much like Writeacher's description):
Spatial: A descriptive paper often is best organized spatially, literally starting with one part of the item being described and moving to the next part and the next part and so on. For instance, a paper describing a car might start with the front end, then move to the engine and hood, the dash, and the front and back seat areas, then finish with the trunk and rear bumper. Depending on the topic — description of a place, building, person, etc. — arrange the details of your essay by describing the item from top to bottom, left to right, inside out, outside in, most prominent part to least, or whatever spatial way seems to work for your topic and audience.
P.S. That came from the 2nd site on this GOOGLE Search: