It's sort of a play on words. As you know, in your DAW there is an adjustable grid, against which notes are placed in time and can be quantized, etc. I love Black Sabbath, and Bill Ward is an inventive and influential drummer who played parts that anchored and defined songs, but I've always felt his timing drifted more than most of his contemporaries (John Bonham, Ginger Baker, Keith Moon, Mitch Mitchell, Nick Mason, etc.)
So, with that in mind, part of his "feel" is "off the grid."
you should NOT line that up with a grid, that's the point. That said though, never been a fan of Ward's playing, Vinny Apice on the other hand, dude's awesome