What is Exposure Bracketing?

Exposure bracketing is a technique where, instead of taking a single photo, you take three (or more) that are all exposed slightly differently; normally one is correctly exposed, one slightly underexposed, and one slightly overexposed. 

When should you use Exposure Bracketing?

Anytime you feel the scene is a challenging one (too much highlights or shadows) as far as lighting is concerned, e.g. sunsets are usually better taken slightly under-exposed so use exposure bracketing there