"Das Mädchen hat Röcke."
Translation:The girl has skirts.
Sometimes you will be given new words in a sentence if Duolingo thinks you are ready for them. For example maybe you've seen the singular "Rock" and are familiar with similar pluralization patterns so the program is asking you to apply the concepts you've learned. Of course if you ever feel overwhelmed, you can just scroll over the new word to check the dictionary.