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.

Man on his death bed, 1714. Credits: Vincent Laurens van der Vinne, collection Rijksmuseum (public domain)