Coplanar waveguides form PCB transmission lines for very high-speed PCB signals.
Impedance control is critical, as signal frequencies are usually very high. The transmission line length on the PCB is often longer than the actual signal wavelength.
Vias bracket the signals to connect the adjacent GND area on the surface to the GND plane underneath the signal traces. The exact placement of the vias is not critical, but care is exercised to keep them close to the edge of the GND plane. Their lengthwise spacing is anywhere from 1/10th to 1/4 of the wavelength of the signal they bracket.
