A file with .qgs extension is a project file created with QGIS, which is an open-source cross-platform GIS application. All the project settings such as layer properties and auxiliary data for the project. It is created when a map project is saved to disc. It is actually a configuration file that retains information such as pointers to the GIS data, styling information about the layers, and project title. QGS files can be opened with QGIS software that is freely available to download.
QGS File Format
QGS files are in XML format and store projects data as XML tags. A QGS file contains information such as:
the layer tree
the map canvas extent
mapview docks (2D and 3D)
the layers with links to the underlying datasets (data sources) and other layer properties including extent, SRS, joins, styles, renderer, blend mode, opacity and more.