Edit Bar controls: none.
This tool allows you to snap picked points to the nearest point on a grid line. The snap mode is only allowed if you are currently in a 3D layer.
The cursor is modified to show "Grid Line" to indicate that the snap mode is active.
The grid can be made visible using Visibility - 3D Visibility - Geometry - Grids.
Related topics: Snap Modes and Lock Modifiers.