When shooting High Dynamic Range (HDR) photos, the photographer must bracket the pictures. “Bracket” is just a fancy technical term which means that you take the same shot with different exposures: typically, one exposure as measured by the camera’s meter (mid tones); a second underexposed to capture detail in the highlights (high tones); and a third overexposed to bring out the detail in the shadows (low tones). Since one picture rarely has the capability to retain detail in the high, mid and low tones, we bracket and then combine the three pictures into one in post processing to obtain a HDR picture.
Depending on the complexity of the lighting situation of your scene, you may choose to take even more than 3 photos, varying the amount of over and under exposure in each photo. You can use the Auto Exposure Bracketing (AEB) function or manually dial in a positive or negative exposure using the exposure compensation function of your camera. Some cameras allow only 3 AEB shots; others 5 or more, and you may be able to specify 1/3 EV, 2/3EV or 1EV (or even finer intervals) over/under exposure with each shot.
The question then becomes, How much do I need to bracket? How much over and how much under exposure? The quick answer is to make sure you’ve obtained all the tonal information present in the scene.
Brian Matiash answers this question in more detail in an excellent article on LensProToGoBlog.