Aperture is the setting on a camera that controls how much light passes through the lens to strike the camera sensor (or film), thus it serves as a brightness control. Aperture is expressed in f-stops such as f1.2, f8, f22, etc.
Exposure bracketing is when you take a series of pictures that have slightly different aperture settings. One reason to do this is if you are not certain what the best exposure is going to be. Another might be if you want to create a HDR photograph. Most cameras have a way to do this automatically, or you can do it using exposure compensation, or by manually adjusting the exposure between photos.
ISO represents the sensitivity of your camera sensor to light. ISO can vary from 100 to as high as 25,000 or more on some digital SLRs and mirrorless cameras. The bigger the number, the more light sensed.