An Archiefvormer is the creator of an archive. Examples of archiefvormers are municipalities, churches, societies, foundations and private persons.
Thinking about which archiefvormers your ancestor may have been in contact with is a great way to discover unknown records. Read the article Find your ancestors by understanding how records are created for more information.
If you check the finding aid (inventaris), you can see who the archiefvormer was. Most online finding aids will allow you to click on the name of the archival creator to see if there are other record groups created by the same organization or person.