Via holes are important components of multi-layer PCB boards, with drilling costs typically accounting for 30% to 40% of production costs. They serve as connections between layers or for device fixation, and are categorized as blind, buried, and through vias. Designers aim to minimize via size for high-speed circuits, considering parasitic capacitance and inductance effects. Strategies to mitigate parasitics include selecting optimal via sizes, using thinner PCB boards, minimizing unnecessary vias, and optimizing power and ground via placement.