The Grids commands on the Tools menu 
Grids And Snaps submenu provide functions to manipulate the home grid and grid objects.
The Snaps system uses grids both as visual references and as physical snap references. In addition, grids serve as construction planes, because the objects you create are built on the active grid in the active viewport.
As a default, 3ds Max uses a construction grid called the 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 orthographic viewport is coplanar (parallel) with one of the grid faces.
A custom grid, or grid object, is a helper object that you can use in place of the home grid. While the home grid is three intersecting planes whose orientation is fixed in world space, a grid object is a free-floating, 2D grid that you can position anywhere and rotate freely, letting you construct objects at any angle in world space. You can also generate grids automatically on existing geometry when creating objects by using AutoGrid.