<p>I have heard this concept used frequently, but I don't have a really good grasp of what it is.</p>

I beg to differ, Wikipedia is pretty clear on this.
In computer science, marshalling
  (similar to serialization) is the
  process of transforming the memory
  representation of an object to a data
  format suitable for storage or
  transmission. It is typically used
  when data must be moved between
  different parts of a computer program
  or from one program to another.
http://en.wikipedia.org/wiki/Marshalling_(computer_science)