That sounds like the document grid, which can be turned off in the view menu.
Peter, thank you. I am most appreciative for your simple answer. There is MUCH iI don't know about InDesign and its use so I miss the obvious. S
The keyboard short-cut 'w' is very useful for toggling between InDesign Normal and Preview views.
But it's very easy, if you’re using the Type Tool, to inadvertently add an unwanted w to a document (by the way, you press Esc to come out of the Type Tool).
I’ve found the way round this is to set up a new short-cut for toggling between Normal and Preview views. On a Mac I use 'alt + w’ and never have any unwanted w problems!