The hash marked lines are the dotted line. As the service area is for you, so the dotted lines are for your opponent. This is the area in which your opponent cannot enter, until the ball crosses that area.
If your opponent steps up too soon, it could cut the ball off. That will be a point for you, and against your opponent because he moves into the safety area which is demarcated by the hash marks.
The yellow colored lines, in the following picture, are dotted lines.