It is a unique alpha-numeric code for each device that is printed on a label that is visibly attached to the trainer. In the case of Pro2 and GO, the sticker is on the bottom right of the plastic base that rests on the floor. In the case of the Air, it is located below the triangular base.
As noted, this code is unique to each unit and allows us to determine the exact date and time of its manufacture. You will have to provide this code whenever you report a problem with your trainer.