Internal conflict would be a conflict in the book or passage between man and himself. For example, a young child is told not to take a cookie before dinner, but he can have one after dinner. He really wants the cookie, but knows he would be disobeying his mother if he took it. So he wrestles with himself as to whether he will or will not take the cookie. It's internal because he is not fighting with anything on the outside, like nature or animal or man.
Sorry it's so long, hope this helps!