This theory goes that some people (mappers) create a mental map of how the world works, and any new knowledge has to be integrated into the map - or if it doesn't fit, the entire map has to be rearranged. Packers just store little packets of knowledge for each situation, without connecting them together.
The mappers therefore can do problem solving by connecting the different parts of their map when they come across something they haven't seen before, whereas packers are will try and apply one of their existing knowledge packets to a new situation, and if none of them fit they are lost.
I think the analogy works, certainly some people I've helped with using their computer don't seem to be able to apply knowledge learned in one program to another one, and need to learn everything by rote instead of figuring it out as they go. So either they don't have enough experience yet to build a meta map/model that would let them figure it out on their own, or they don't even try to.