.In this settings you can able display a Grid on screen for visual
reference. Also you can specify the spacing of grid. To make Grid on/off
go to Settings->Grid menu and click Grid on/off or click on icon
present in Snaps toolbar as .
Grid Size: To specify grid spacing go to Settings->Grid menu and
click Grid-Size or click on icon present in Snaps toolbar as 
. After doing this you will get a dialog box as shown below.
.
In this dialog box you just specify value for X and Y as shown there. Also you can make grid invisible by making uncheck in Display Grids in dialog box