The document discusses the Esri-Leaflet plugin, which adds ArcGIS functionality to the open-source Leaflet mapping library. It allows users to add ArcGIS basemaps, feature services, symbols, and other widgets to Leaflet maps. The plugin is open-source and extends the Leaflet API and namespaces to support ArcGIS Online services. Examples are provided for common tasks like adding basemaps, feature layers, popups, and performing queries.