A gap in a drawing is a condition that exists when two lines intersect graphically in the drawing but not physically in the plant. In order to portray this condition, you can choose that lines show a gap at these non-physical intersections. This can help the clarity of your drawing considerably.
Gapping is carried out automatically when you open an existing drawing. If you want to perform gapping on a drawing that is already open, you must use the command Tools > Gap Now.