The Grid filter places a custom grid over the image. All aspects of the grid can be customized.
Grid Settings
The Horizontal, and Vertical settings are used to adjust, Width, Spacing, and Offset. Width sets the line width when rendering the grid. Spacing affects the distance between the grid lines. Offset determines the offset from the top-left corner of the image, from which rendering will begin. Located below the Offset settings, are located the Color Wells, which set the colors for the grid lines. All of these settings can be "Locked" together to balance the Horizontal, and the Vertical grid lines, or can be "Unlocked", to provide more control.
The Intersection settings add another dimension to the Grid filter, by creating an intersecting grid that begins within the bounds of the primary one.
Width works much the same as the primary grid settings.
Spacing affects distance that the intersection grid starts from the point of origin.
Offset works by extending the length of the rendered lines.
One final set of options are the Units dropdowns, which can be adjusted to reflect different measurement units for the grid lines.
This button updates the preview window so any grid changes can be seen prior to the final render.
The three color wells set the color for Horizontal, Vertical, and Intersection elements.