"The beds are dirty."
Translation:Die Betten sind schmutzig.
I recommend using Duolingo as a web page instead of as an app. On the webpage, after you select a lesson but before you click 'start', you can scroll down to read tips.
Basically, though, every noun in German needs either der, die or das - corresponding to the grammatical gender of the word (a concept that doesn't exist in English, but does in many other languages). It's usually got nothing to do with any concept of biological gender, and you just need to remember which one it is when you learn a new word. Conveniently or confusingly - depending how you look at it - all plurals use die as well, no matter what the singular word uses.
That explanation is for nominative case, i.e. when the noun is the subject of the sentence (the thing that does the action). There are other cases in German too once the sentences become more complex, but there are tips pages to explain them too.
das Bett - singular, neuter noun
die Betten - plural noun