An ambtenaar is a civil servant or government employee. In civil registration records, you will often see an ambtenaar van de burgerlijke stand (clerk of the civil registration) officiating the marriage or registering the birth or death.
During your research you can encounter ambtenaren in two ways: your ancestors may have been ambtenaren themselves but they also would have dealt with ambtenaren like clerks, teachers, soldiers, police officers or town criers in their every-day lives. The records created by or about ambtenaren can be found in the archives of the government agencies they worked for.