Onbekend means “unknown.” Niet bekend means “not known.”
You may encounter these terms when informants are asked for information they do not possess. For example, in death records, you may see a reference that the names of the parents are not known to the informants. Informants for death registers were often neighbors, who may not have known the deceased’s parents.
Other records may mention “vader onbekend” [father unknown] if the child is born out of wedlock. For tips on researching those cases, see How to find the father of an illegitimate child.