The valley 'flows' northwest out of the hilly Margaritifer Terra, where the Iani Chaos depression and 3° North. Ares Vallis was the landing site of NASA's Mars Pathfinder spacecraft, which studied a region of the valley near the border with Chryse in 1997. It has been argued that Uzboi, Ladon, Margaritifer and Ares valles, although now separated by large craters, once comprised a single outflow channel flowing north into Chryse Planitia. (wikipedia entry)
A vallis is a valley. On Mars large valles are named after words for Mars/star in various languages. Small valles are named after classical or modern names of rivers.