Many software programs use a series of horizontal and vertical lines across the screen as a "grid." Often these programs will have a setting that allows "snap to grid" to set certain points on the screen in an evenly spaced manner. The spacing between the grid lines is called "grid spacing."
Grid spacing can be measured in either pixels or inches. A pixel is a single dot on the screen. Higher resolution screens have more pixels per inch, i.e. each pixel is smaller, so more detail is available.
Variable Grid Spacing
In addition to allowing you to choose between pixels and inches, some software also allows you to change the spacing between the grid lines. For instance Microsoft Visio in "variable grid mode" allows you to choose between "Fine," "Normal" and "Coarse" modes. Coarse has the most space between grid lines, while fine has the smallest space between grid lines.
Snap To Grid
Drawing programs, like CorelDraw or AutoCAD, have a "snap to grid" feature that forces dots you draw to jump to the closest intersection of the invisible grid lines. This makes it easier to draw straight lines, but makes it more difficult to get precise placement of objects should you want them to be located between the grid lines rather than on the lines. Therefore, generally the software will allow you to turn "snap to grid" on and off.