The white dashed lines on the Aeronautical Map outline Air Traffic Control sectors (ARTCC and FIR). If you zoom out, you can view the controlling agency's name.
Like Terrain, Airways, and Airspace, they can be turned ON or OFF with the Map Filter buttons or within the Map Settings menu.
Image 1. ForeFlight ARTCC labels and boundaries.
Image 2. FAA ARTCC labels and boundaries.