When we speak of gather and fullness, we are talking about the width of material in each curtain. You may notice that most window curtains do not lie flat. Traditionally nets, voiles and curtains are hung so that they fall into soft folds.
The folds, also known as 'Gather' or 'Fullness' are created by increasing your width 2x to 3x your flat window reveal size.
Gather provides greater privacy and insulation as well as for aesthetic reasons.
If you prefer a flat panel against the window, you can select 'Flat' as the gather.