This is a type of projection for mapping a portion of the surface of a sphere (or the whole sphere) to flat images. The images are arranged like the faces of a cube and are each one in the Rectilinear Projection. This cube is viewed from it's center.
Four cube faces cover front, right, back and left, one the Zenith and one the Nadir, each of them having 90°x90° Field of View. In each cube face all straight lines stay straight, hence it is very good for editing. See Extracting and inserting rectilinear Views on different possibilities how to extract cubefaces from an equirectangular image and inserting them again.