When you isolate a polygroup in ZBrush, you can then work with it independently. For example, you might select Tool > Geometry > Del Hidden to delete the polygroups that are hidden after you’ve isolated one.
Hold CTRL + SHIFT and click on a given polygroup to isolate it, hiding all the others.
Hold CTRL + SHIFT and click on an empty place in the canvas to return all polygroups to view.
If the Select Rect brush is your active brush, you can SHIFT + CLICK and drag a rectangle on an empty place in the canvas to invert the visible polygroups.