Grid preferences control the background grid - the light grid behind your diagrams. Grids can make drawing many images much simpler.
The grid size can be set using any measurement. You can type 1cm, 0.25" or 50pt.
Ticking isometric creates an isometric grid that makes drawing isometric diagrams much simpler.
Show grid turns the background dots on and off.
If you are having trouble seeing the background grid (on an interactive whiteboard for instance) increase the opacity so that it becomes more obvious.
Drawing Isometric Diagrams Video