This is a mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release.
The sky can be thought of as a sphere that surrounds us in three dimensions. To make a map of the sky, astronomers project it into two dimensions. Many different methods can be used to project a spherical surface into a 2-D map. The projection used in this image of the sky is called an “equirectangular.” This method projects the sky into a rectangular shape with Cartesian coordinates, this projection is useful for planetariums that may wish to display the image on their domes.