Modeling, in terms of NLP is learning how to think and behave in the same way that people who are achieving the results you want think and behave.
Believe it or not, you were actually born modeling behaviors. As a child you modeled your primary caretakers and that is how you learned to develop your first abilities in life; walking, talking, and whatever else you picked up on.
NLP Modeling is taking that same concept and utilizing it to your advantage.
Modeling is based on the premise that success can be broken down to a step-by-step strategy. If someone else has had success at something, then anyone can apply the same strategy and achieve the same results.