A standing wave ratio (SWR) is a measure used in radio frequency systems to evaluate the efficiency of power transfer between a transmission line and an antenna. It indicates how well the antenna is matched to the transmission line, with a lower SWR value signifying better matching. An ideal SWR is 1:1, meaning all the power is transmitted without reflection. When the SWR is higher than 1:1, it indicates that some power is being reflected back towards the source, which can lead to signal loss and potential damage to equipment. Understanding and optimizing the SWR is crucial for effective operation in systems involving antennas and transmission lines.