Because dere /=/ they. It means "you all" or "you (plural)". If you were addressing a class of children you would say "Dere har et eple." (You all have an apple). But if you pointed at someone out of the window who was holding an apple, you would say "De har et eple." (They have an apple.)
Du har et eple. = You have an apple. (addressing one person)
Dere har et eple. = You (all) have an apple. (addressing more than one person)
Norwegians can obviously easily tell the difference by the pronoun. In case you have to translate "You have an apple." without a given context both answers ("Du..." and "Dere...") are correct and accepted by Duolingo.