An 'icon' is the best route at that grade in the area. Wherever you are in the index there can be only one icon route per grade.
Crag Guru determines iconic status based on a number of factors including number of ascents, quality ratings and desirability (how often the route is flagged on someone's hit-list).
Icons change as you navigate through the index. For example at a country level it shows the best route at each grade in the country, at the state or province level it shows the best route at each grade in the state or province. There is also a minimum threshold that a route must achieve in order to be considered an icon. This means that as you go lower in the index there might be some grades with no icon route or even no icon routes at any grade.
At a route page, icons will be drawn from all routes at the top level crag.