Translation:The grandparents have many grandchildren.
En besteforelder: A grandparent
Besteforeldren: The grandparent
Besteforeldrene: The grandparents
Norwegian (along with, AFAIK, all the rest of the North Germanic languages) consider "the" to be part of a conjugation of a noun. A simpler and more standard example:
En katt: a cat
Katten: the cat
Kattene: the cats
Or, in general:
en @: a @
@en: the @
@ene: the @s
"Grandparent" is a bit weird because the base word ends in "er". In that case, all conjugations first swap the "e" and the "r", then append the usual suffix. For example:
en @er: a @
@ren: the @
@rene: the @s
Note that this applies only to en-nouns (I forget the name offhand) - et-nouns behave a bit differently.
datterdatter/sønnedatter = granddaughter (lit. 'daughter's daughter' or 'son's daughter')
sønnesønn/dattersønn = grandson (lit. 'son's son' or 'daughter's son')
I don't know how commonly used those are outside of conversations about genealogy, though. Barnebarn is what I've heard the most often for grandchildren of any gender.