Now this one is a bit of a head spinner when getting into the depths, but the best description is below from the Less Wrong Wiki:-

The ‘map is not the territory’ metaphorically illustrates the differences between belief and reality. The phrase was coined by Alfred Korzybski.
Our perception of the world is being generated by our brain and can be considered as a ‘map’ of reality written in neural patterns. Reality exists outside our mind but we can construct models of this ‘territory’ based on what we glimpse through our senses.

Source: Less Wrong Wiki

Read in much more detail here with Farnham Street – The Map Is Not The Territory