Wikipedia
The Ahnenpaß (literally, " ancestor passport") documented the Aryan lineage of citizens of Nazi Germany. It was one of the forms of the Aryan certificate (Ariernachweis).
The term Aryan in this context was used in a sense widely accepted in scientific racism of the time, which assumed a Caucasian race which was sub-divided into Semitic, Hamitic and Aryan (Japhetic) subraces, the latter corresponding to the Indo-European language family; the Nazi ideology limited the category Aryan to certain subgroups, while excluding Slavs as non-Aryan. Nevertheless, the de facto primary objective was to create extensive profiling based on racial data.
The investigation for lineage was not obligatory as it was a major undertaking to research the original documents for birth and marriage. Many Nazi followers had already begun to research their lineage even before law required it (soon after the NSDAP took power on 30 January 1933).
One important law which was issued on 7 April 1933 (after the Nazi assumption of power) was called the Law for the Restoration of the Professional Civil Service, and it required all public servants to be of Aryan descent. The law, however, did not define the term "Aryan" and a subsequent regulation was issued on 11 April as the first legal attempt by the Third Reich to define who was, and who was not, a Jew. Germans aspiring for the document had to prove they didn't descend either from Jews or Slavs. The Ahnenpass was given out to citizens of other countries, under the condition that they were of "German blood" and it itself stated that "wherever they might live in the world" Aryans could be citizens from German minorities living in countries like England, France, Czech Republic or Poland.
The implementing decree followed the pre-Nazi trend found in the Aryan Paragraph and read in pertinent part that:
The applicable fields were later enlarged under different laws to include lawyers, teachers, medical doctors and finally requiring a proven Aryan lineage even to attend high school or to get married. Usually, the lineage was investigated four generations back. The Ahnenpass cost 0.60 Reichsmarks.
The Ahnenpass was not public record; the document was shown where required and returned to the bearer.
As a result, genealogical research particularly flourished in Germany during the Third Reich. Opposition clergy helped many racially persecuted individuals by providing them with fake passports as a personal document necessary for survival.