Slate gives the background on this map, but essentially it shows how the Europeans took land away from Native Americans.
While the time-lapse function is the most visually impressive aspect of this interactive, the "source map" option (available on the map's site) offers a deep level of detail. By selecting a source map, and then zooming in to the state you've selected, you can see details of the map used to generate that section of the interactive. A pop-up box tells you which Native nation was resident on the land, and the date of the treaty or executive order that transferred the area to the government, as well as offering external links to descriptions of the treaty and of the tract of land.
When dealing with semi-nomadic tribes, Saunt added, negotiators sometimes designated a small reservation, "rather than spelling out the boundaries of the cession."
This vagueness benefited the government's purposes in crafting treaties and executive orders. "Greater legality and more precision," Saunt argues, "would have made it impossible to seize so much land in so short a time."
You can find the interactive map here and it's well worth spending some time exploring the different layers. It may make you sad to realize that how much of this history is not taught in your history classes... unless you seek it out.