When showing the index of a trip, you can specify the locations you visited using a custom map. Usually custom maps are cleaner than using some other solution like google maps. Let's say you visited Madrid and Paris during a trip in Europe. You'd start with a blank map of europe, for example:
Then on this map, you would find the (x,y) coordinates of Madrid and Paris, and would specify those coordinates in the trip configuration file. The end result of the process would be that when the trip index is shown, the locations you visited are "pinned" by placing on them a clickable image:
In this case, the 
icon is used to show the locations that you visited - the specific icon depends on the chosen skin. (The image "map_pin.gif" of the skin folder is used)