It means an antenna with a known gain at a particular frequency. It is calibrated by knowing all the parameters in your loop. I.e., signal generator output power, cable loss, transmitting antenna gain, pathloss, antenna-to-be-calibrated, cable loss, received power. You know everything in your loop except the antenna-to-be-calibrated gain. You then use simple math to find the unknown antenna's gain. Each frequency will have its own values around the loop, so the unknown antenna will have to be calibrated at all the frequencies that you want.