I will now try and express what I know of the "theory" of primitive accumulation. If you ever feel like further reading you can read chapter 33 of Capital. It is the easiest part of the book and you can then tell people you have read the book.
Adam Smith started from the premise of Robinson Crusoe, Marx tells us. Smith proposed that the political economy existed from time immemorial, whereas we know that no man is an island. Marx contended that at points in history great swathes of the productive forces, most notably the Earth and human labour, i.e. humans, were expropriated and placed in private hands.
The process of primitive accumulation has continued right up to this late modern period. The massacre of Darfur springs to mind. The fact that remnants survive of the way of life that existed under forms of what Engels (social-democratic buddy/sponsor of Marx) described as primitive communism enables us to study and perhaps even preserve these as a foundation for the future.
The process of primitive accumulation is partly the process of creating free labour, i.e. freed of ties to the land that once sustained us. The acts of enclosure and the highland clearances are the most notables instances of primitive accumulation in England and Scotland respectively.