Mirror mode in a backup camera refers to the way the camera image is displayed; specifically, it shows a laterally reversed image (left is right and right is left). This simulates the view you would see in a traditional rearview mirror.
Essentially, mirror mode ensures that what you see on the backup camera screen corresponds to your natural understanding of spatial relationships when looking in a mirror. For example, if an object is approaching from the left side of your vehicle, it will appear on the left side of the backup camera display. Without mirror mode, the image would be reversed, which could be disorienting and make judging distances and avoiding obstacles more difficult.
Most backup cameras are set to mirror mode by default. The linked Chevrolet video demonstrates how a rear camera mirror works in a vehicle, illustrating this function. Some vehicles may allow you to toggle mirror mode on or off in the settings.
