It's a Vietnamese soup. You start with a broth, then add vegetables, noodles, and/or meat.
Other flavorings can be added as well. It comes in a great big bowl--it's the most fabulous meal. Very inexpensive, too. Each restaurant will have something like 20 variations of this soup. I first had it about ten years ago in Northern Virginia, where my parents live. There are lots of different Pho restaurants in their area. None by me, sadly, although there are probably some in Chicago.