Bracketing is where you take a picture with "normal settings" (speed, aperture, ISO, White Balance, Focus, etc.). Then you (or the camera) changes one of the settings and takes the picture again. Then changes the setting to the other side of "normal" and takes the picture a third time, etc...
For example, you take a picture at 1/30 of a second, and set the bracketing script to take a picture at + and - 1 Fstop of 1/30 of a second (1/15 of second and 1/60 of a second in this example). And you can set the script to take another set at +/- 2 Fstops, etc... In theory, you can do that with any adjustable parameter. This allows you to ensure that at least on of the shots does not have blown highlights (or dark areas). You can also do this for "HD" pictures (where you mix multiple exposure values so that one picture can have, for example bright lights and deep shadows with details visible).
Av equals "aperture value" (Fstops) and Tv equals (I guess) "time value" (shutter speed like 1/30 of a second).
-Bill
