An internal representation is what we are seeing, hearing or feeling within our mind, typically in response to something that happens around us or us us including what we see, hear, feel, taste, smell - and also to things that happen within our minds such as memories, thoughts or desires.
That internal representation can itself be something that we see within our minds or a feeling we get, or dialogue that we hear or say internally.
Here is an example of an internal representation. Try doing the following simple thing which will only take you an instant then go onto the line after for some more information.
Think of someone you like.
Now you have got that person in mind, you may or may not be aware of your internal representation of them, for some of you you may be able to imagine a picture of them, for others the thought of that person may invoke feelings or you might get the idea of words associated with that person. They are all internal representations. For others you may not be aware of these things or you may not be able to bring them to mind yet, or ever.
It can be useful to be aware of internal representations to use NLP to develop yourself as they were a key part of the development of NLP.
Understanding internal representation will help you to go on to understand representational systems of your self and others.
Link to www.cleanlanguage.co.uk and more about representational systems.