Intuitively, bracketing requires at least 3 images in order to capture the full spectrum of colour detail. This would require the photographer to take a number of shots of the same subject or composition, but using different aperture f.stops for each photo.
The aperture f.stops determines the amount of light allowed to reach the camera lens. So for example, 3 identical photos with 3 different f-stop ranges of exposure;
a) the mid-range exposure,
b) the highlights, and
c) the shadows.