A functionality of the QGIS Print Composer.to generate series of maps with a common layout and dynamic content.
Generally speaking, an atlas is a collection of maps. However, this tag refers to the specific QGIS tool used to create map books, or sequences of related maps whose extents are based on features in a reference layer such as a grid. It is similar to Data Driven Pages (or its predecessor, DS MapBook) in ArcGIS.
More information, including a tutorial, is available in the QGIS User Guide available on the documentation page.