This is the first of the 22 NLP presuppositions...
This presupposition states that our experience of reality is not always exactly the same as reality itself. What we hear, feel and see is based upon previous experiences and how they have been connected or wired within our brain. So, for example seeing a dog will not necessarily be perceived as it is but will be based upon previous experiences of dogs. This means that even the most gentle dog in reality can be perceived as vicious by someone who is frightened of dogs and so on.
The process of connections or wiring (neuro nets) is influenced by internal filters (see NLP Communication Model) that subtly change or alter the 'history' or 'reality' of an event, thus creating a different internal memory of it or of what happened. In other words the human mind creates variations of reality and does not record or register the reality itself.
Internal mapping accounts for why one person's pleasure can be another person's pain, why one person's fascination can be another's boredom. More importantly it is why two people with a similar education, upbringing, opportunities and circumstances can achieve such diverse results in their lives. It is not the 'territory' with which we interact but our filtered 'perception' of the territory, or our map. In other words, just as a map of Los Angeles is not actually Los Angeles, so our experience of reality is not reality itself.