The colored dots show map elements on the OpenStreetMap that are tagged with an 'amenity=replaced_later'-tag. A purpose of the map is to visualize how these elements have been edited over time. The colors of the dots indicate the date each object was last edited. For example, an orange dot represents a map element that was last edited in 2009. Moreover, the slider on the bottom of the screen can be used to select elements that were last edited during a specific time period. Clicking on a marker also displays further information about the map element.
The underlying map represents the current OpenStreetMap, and the marker data uses OpenStreetMap data from the 10/2015 snapshot. The OpenStreetMap data is © OpenStreetMap contributors, and available under the terms of the ODbL. More information is available on this visualization's Github page.