A printed circuit board (PCB) made using a material which is a high-performance laminate often used in high-frequency applications, while "FR4" stands for "Flame Retardant Level 4," a widely used standard material for PCBs composed of glass fiber reinforced epoxy resin, indicating that the board is made with a flame-retardant material; essentially, it's a high-quality PCB utilizing a specialized material from the Rogers company, with the added benefit of flame retardancy due to the FR4 standard.