Anyway, it basically means that they plan to open all the boxes and empty the contents onto the table. If you "dump" something out of something else, or onto something else, then you tip them out.
I have a large cardboard box full of pens and pencils. I specifically want to find the purple pen but I can't see it, so I decide that the easiest way is to empty all the pens and pencils onto a table so that I can see them more clearly. I might say "I'm going to dump all the pens and pencils onto the table" or "I'm going to dump them all out of the box [onto the table] so I can look through them."
It suggests that I am simply going to get the box, tip it upside down so all the pens and pencils fall out at the same time. I'm not going to carefully take the contents out one by one.