There is a brief overview of the concept here, but the original and IMO definitive work on the subject (it was Bandler's masters thesis IIRC) is The Structure of Magic, Volume I. It's not too hard to find a copy electronically if you can't find one physically.

As the above-linked page says:

The meta model consists of categories of questions or heuristics which seek to challenge linguistic distortion, clarify generalization and recover deleted information which occurs in a speaker's language.

In the book, IIRC, there was more of a discussion about how the maps in our heads are created by distorting, deleting, and generalizing information from the territory. The meta-model is an attempt to codify how these distortions, deletions, and generalizations are reflected in our language, and provide a set of tools to allow someone to reconnect their map with the territory, to identify where the map needs updating in relation to a problem.