Grids are used both as visual references and as physical snap references by the Snap system. In addition, grids are construction planes, since the objects you create are built on the active grid in the active viewport.
As a default, there is a home construction grid called home grid. The home grid is the basic reference system, and is defined by three fixed planes on the world coordinate axes. It provides ready-to-use construction planes, intersecting at right angles through the origin. When the home grid is displayed, each is coplanar (parallel) with one of the grid faces.