Pickling is serializing arbitrary object-oriented data structures. That is, converting them into a byte stream for storage, transmission over a network or iterative navigation as in this case. It is a more complex process than you might imagine. If you want more information, search the web for information on Python's pickle module.

Does anyone have experience with infamous pickler fetch which shows up in explain plan on index organized nested tables (Nested objects)? It's obvious that someone in internals has a good sense of humor. I guess this is the way Oracle dereferences the embedded collections?