Mastering is the process by which the master recording is transferred to the lacquer master disc.
The process can be a flat transfer where the master is transferred without changes to the lacquer master. Sometimes changes are necessary because there are limitations on what can be transferred to a lacquer disc. Excessive high frequencies will be attenuated and excessive low frequencies will prevent a loud record. Out of phase low frequency signals will cause skips. Changes can also be made to improve the sound.
The master is played into a disc recording lathe which cuts the groove on the lacquer master. A cutterhead mounted on it moves laterally across the lacquer master as it rotates on the turntable. The cutter head holds a stylus which vibrates and cuts the groove.