Ab intestato means “without a will.” The ab intestato heirs of a person are the people who would inherit by law if the person did not have a will. It is a Latin term you may find in Dutch documents regarding someone’s estate, for example in a will or estate division.
If a person writes a will, sometimes they may refer to the ab intestato heirs to indicate what (if anything) was left to them. It is just a shorthand for saying “to the people who would have inherited had I not made a will.” If an estate division mentions ab intestato heirs, that is usually an indication that there was no will and that the heirs were determined by the law of the time and place.