It's a quote from a very famous poem, which you can read in the link that Helen posted. – It seems the text actually says snön lyser vit på taken but it's often quoted as ligger.
PS actually you shouldn't read it, but see/hear it read in the classical version. I found a clip of it which even has English subtitles: https://www.youtube.com/watch?v=0rJ0Ec-jYg8
Hi Bicee - I'm not exactly sure what your question is (I understand that English is not your first language so I'm not criticising your English) but yes, I'm not sure what it is that you mean. But it's possible that you're asking why such an obscure phrase is found on Duolingo, when we would never expect to use it... To answer that, I have always understood that Duolingo basically trains us in the use of words in that language, in general. No, I will probably never say "I am a penguin" in Spanish, but now that I remember "Soy un pinguino!" I can remember the structure for how to say "I am a lawyer" (Soy un abagado!) or whatever it is I need to say. If you want to learn exact phrases, I'd recommend a phrase book or something, but Duolingo is great for getting us accustomed to the language.
Laying is a transitive action, as you might lay a blanket on something, for instance.
Lying is a static action, as a carpet might be lying on the floor.
The some distinction works between lays and lies as well. As noted above, I know it's not uncommon to use them interchangeably in colloquial speech, but it is usually considered incorrect usage, and so we make a difference in the course.
I have to say that the new robot's speech (October 2020) is much more difficult to decipher than the old one's. I simply could not hear the "t" in the word "vit" in this sentence. She made it sound like "vi", which obviously couldn't be correct. She was unclear throughout this whole module, whether fast or slow, and I have not liked her ever since she took over from the previous one. En riktigt sämre robot enligt min åsikt.