Translation:a garage and a house
It's the nominative indefinite ending, -un, which is rarely written, but if it were, it would be written as ٌ
كَراجٌ وَ بَيت
Notice how the small symbol ٌ above the ج looks like two small وو linked together. The grammar behind when we use what when talking about indefinite things is a bit too involved to explain here, so what I suggest you do is pay really close attention to what you hear while following the Arabic letters with your eyes, and you'll eventually get a feel of what to use when. It takes a long time to acquire this skill, so just be patient and listen to a lot of Arabic while following text every day. You'll get there eventually :-)
PS: We also often drop the -un sound when reading out loud, so don't worry too much about it.
Thanks for the information and the positive attitude. From what I know, it's not THAT important to worry about diacritical marks at this stage, unless they change an actual letter (eg ra vs za sound, and da vs dha sound.) That being said, Duolingo needs to increase the font size, especially if this stuff DOES become important in later lessons.