A ghost grid is a guide for organising thoughts, information and sketches, that doesn't compete with content.
Content itself can also be the ghost grid to highlight changes and help draw comparisons — as when you might sketch options for extending a house or designing a garden.
A grid is useful for structure and creation but needs to fade back from content or disappear when no longer needed. Like freeing data from its data prison, a ghost grid emphasises content and information without non-data-ink taking attention.
I learned the term ghost grid from Edward Tufte's Seeing with Fresh Eyes: Meaning, Space, Data, Truth.