Denver is the capital of and largest city in the United States state of Colorado. It is nicknamed the "Mile-High City" because it is a mile above above sea level in elevation.
Olivia Benford wants the Benfords to move away from Los Angeles to Denver so that they can get away from the stress and dangers of the Mosaic Investigation. She mentions that there are two hospitals there where she could work, as well as an FBI regional office where her husband Mark Benford could work. Mark does not want to move, however. ("Better Angels")