GXP is a JavaScript SDK that builds on GeoExt and OpenLayers to provide a complete toolkit for developing complex web mapping applications with minimal configuration. It aims to simplify the creation of web GIS applications by handling UI components, maps, custom components, and the development lifecycle. The document discusses what GXP is, its place in the technology stack, why it is useful, learning resources, and provides an example of creating a full web GIS application from start to finish using the GXP toolkit.