It refers to collecting the 2D slices of a 3D object and arranging them into a orderly output.
Imagine, if you will, slicing an apple into 50 slices so that you can unfold it into thin chips. These are the files in question.
Now imagine that you wanted to paint a fruit basket but you just cut the last apple.
Now have a pile of apple slices without any order that you're trying to arrange.
You decided to go for a walk and take your mind off of your problem, however your girlfriend makes a fruit salad by throwing a sliced pear into the mix.
You wish that you had a way of identifying the apple slices from the pear slices because you need to fold these seemingly identical objects back into their original form.
What you need to map them so that you are able to place them correctly. You know what the fruit looks like but arranging it is impossible.
I'm suggesting a knife that will cut the fruit and mark each slice with an identifying number. Thus giving you a scientific way to recreate a 3D apple from 2D chips.