In this youtube video of "How to Shoot a Basic HDR Photo" from the channel 'Triggertrap How To', at this point what is the person who's doing the lesson, doing?; he rotates the dial on the camera and on the screen, two lines go away from a center line...
The user in the video is using a specialised bracketing function on a Canon body, called Auto Exposure Bracketing (AEB) (complete description from Canon here)
In the "exposure" setting, you can rotate the dial and then 3 lines will appears. Each line represents the exposure compensation that will occur for the 3 photo that you will take immediately after. The first photo will have a "correct" exposure (0 EV correction), the second will be underexposed (line of the right: -X EV) and the last will be overexposed (line on the left: +X EV).
What the user is choosing when rotating the dial is the amount of exposure correction ("AEB Amount") between the first and uncorrected image and the over/under-exposed one (it is symmetric: the "X" in "-X EV" and "+X EV" is the same value).
For example, if the scene has a high dynamic range (very dark areas and very bright ones), you will probably need to capture something like a 15 stop range and your camera can only capture something like 11 stops of dynamic range. You may want to use some HDR technique. By setting an AEB amount of +/-2 EV, the 3 bracketed images will capture something like this for a scene that ranges from EV 3 to EV 17 (EV 3 to EV 17 inclusive is 15 stops) and the base exposure value is set at EV 10.
- "0" Exposure centered on EV 10 captures details between [ EV 5 to EV 15 ]
- "-2" Exposure centered on EV 8 captures details between [ EV 3 to EV 13 ]
- "+2" Exposure centered on EV 12 captures details between [ EV 7 to EV 17 ]
With those 3 images, you will have a dynamic range of around 15 EV (The 11 EV of a single frame plus two more in each direction for the shifted exposures: 2 + 11 + 2 = 15). See also Are RAWs in bracketed exposure (mostly) redundant?
Different Canon models are capable of different numbers of shots in a bracketed series (anywhere from 3 to 7) as well as the maximum step between each shot (usually either +/-2 or +/-3 stops in 1/2 or 1/3 stop increments). Some Canon cameras are capable of doing a series of up to 7 bracketed shots at up to 3 stops per step between shots. For more, please see: Can the 5DS-R take more than 5 bracketed shots spaced by 3 EVs?