High-frequency PCBs are developed from antennas, but the antennas are not the loops you see anymore - two antennas are embedded in the circuit board as etched copper structures.
So we can say that a high-frequency PCB is an antenna PCB that generates, evaluates, emits, and receives radio-frequency signals of more than 1GHz.
To better understand what a high-frequency PCB is, you should know its basic structure.