SVG stands for Scalable Vector Graphics and are the way that points, lines, and polygons are drawn on a browser. It's the same process that is used in programs like ArcPro to draw these shapes and act as the foundation of any polygon-based features when working with browser coding languages, which is what Observable uses.

From this map, it seems that the percent of impoverished people that are under 18 in Illinois doesn't have a strong spatial pattern. There are clusters of counties with high proportions of children in their poor populations, such as the center of the state and the southeast of the state, but the overall trend across all the counties is not obvious by just looking at this one map.