WHAT IS BRACKETING?
It's a feature inside the camera and it works in various ways. It gives you an image with multiple exposures in the same subject in the image.
In a high-end camera, there will be a (Bkt) button on the left-hand side of the camera and for others, you will get this setting by pressing the (i) button in the camera.
Here are a few examples so it will be more clear to you.
This way you end up with the three images in a series with exactly the same composition but at different exposures for you to select the best of later on.
WHEN TO USE BRACKETING?
Most of the time if you get confused about what exposure will be needed in the particular frame, at that time you can use bracketing to get the perfect exposure. Its start with three to nine different exposure variation. for ex. AE0.3, AE0.7, AE1.0, AE1.3, etc.‘Automatic Exposure Bracketing‘ (AEB) can be useful while shooting the sunset because it changes from moment to moment) and other times you can use it manually.