AE Shift stands for "Automatic Exposure Shift."
It is probably one of the easiest (in-camera) ways to make your image brighter or darker.
By adjusting this in the positive or negative values, the camera automatically changes all the settings to create a brighter or darker exposure.
So, AE Shift will change the shutter speed, aperture size, and gain, if it has to. Typically, videographers try to avoid too much gain, because it makes the image rather fuzzy and poor looking.