Bad breath! It is caused by harmful bacteria living in your dog's mouth (and their gross waste products). When those bacteria get together and form a film on your dog's teeth, it's called plaque.
Sometimes, bad breath can be caused by more serious problems than surface-level bacteria. Gingivitis, tooth decay, and other infections can all cause bad breath, and humans should partner with their dog's vet for help and treatment.
Bacteria! Specifically, the acids created by bacteria as a byproduct of eating the sugar floating around in the mouth.