What does het achter huis mean in English?

Answer 1

het generally means the, or a third person "it", etc.
The hiding place behind the bookcase where Anne Frank and her family hid is known as the "Achterhuis".
It is Dutch. If you are looking at a Dutch copy of Anne Frank's diary, it it titled "Anne Frank: Het Achterhuis". achterhuis is one word :)

Answer 2

"Het achter huis" in English means "the back house" or "the rear house."