A choropleth map of the world colors each country according to its population, using a threshold color scale with a sequential red palette. Hovering over a country dims others and reveals a tooltip with the country name and population in millions, while clicking zooms the map. Population data is joined to GeoJSON country outlines via three-letter country codes, and the map is rendered as SVG using `d3.geoRobinson`, `d3.geoPath`, and `d3.scaleThreshold` with an animated `transition` on opacity and stroke.