Translation:eating and sleeping
It's indicating they're "sun letters"; 13 of the 26 letters cause the ل in ال to not be pronounced, but instead assimilate with the following consonant.
ن is one of those letters, so here it's pronounced "an-naoom" rather than "al-naoom"
Hopefully they're still adding notes to the lessons and they'll cover it soon, but there's more information here.
Thank you, AnUnicorn, for reminding us about assimilation and about sun and moon letters. Duolingo mentioned them in the Tips of this chapter (Hobbies), which, I think, has a HUGE amount of new stuff. But I found an incredibly helpful article in Wikipedia, which pointed out that the difference between sun and moon letters wasn't random, but depended on whether they were coronal or not. I looked up "coronal" and found it meant something like "involving the tip of the tongue", so T, D, L etc are coronal (sun letters). This helped me a lot. Not to have to learn two random lists of consonants. Ah, I've just found it. Surprisingly enough, the article is called "Sun and Moon letters".
Yes, there are 28 letters, or 29 if you count alif (ا) as being separate from hemzah (ء) as we do here for categorizing sun/moon letters. As you'll see, however, alif (ا) is not included in either list so it's the remaining 28 letters that are indeed divvied up into sun/solar and moon/lunar category with 14 letters in each category. The 14 moon/lunar letters are compiled into the below phrase and it's important to note that the first letter in the phrase is actually a hemzah (ء) and NOT an alif (ا), per my previous explain:
إِبْغِ حَجَّكَ وَ خِفْ عَقِيمَه
The other 14 letters are, of course, sun/solar letters by process of elimination--including the letter laam (ل) itself which many lessons ask about, though it effectively sounds more or less the same either way.