Image training is a specialized form of training in the Dragon Ball series.
When image training, the fighter involved closes their eyes and pictures a foe, they then proceed to fight them as they would in a real battle. Two fighters can participate in image training together by linking their minds. Fighters can get stronger through image training.
Image Training is useful as it allows one to train practically anywhere thus it is useful when confined (such as Frieza while cocooned in Earth's Hell) or in an area that would prohibit a normal training regimen (such as the Nameless Namek's Spaceship).
In Dragon Ball Super, in the manga Goku undergoes image training in his radish field prior to Beerus' arrival. He faces final form Frieza and defeats him with one blow as a Super Saiyan. He then faces Perfect Cell and quickly defeats him. He prepares to face off against Kid Buu, but the training is interrupted before he can.
In both the manga and anime, after his death during his attempted revenge, Frieza underwent image training while in Earth's Hell, battling and killing Goku and the other Z Fighters thousands of times. Frieza's power and energy control increased greatly thanks to this.
In the anime, after returning to the present, Future Trunks underwent image training against Goku Black, but was unable to defeat him in any of the sessions. Shu demonstrates an awareness of image training giving a brief explanation of it to Mai as he and her observed Future Trunks' image training.