I have a hole in my back tooth that smells so bad. What is it?

Hole in tooth. It's either a decayed or infected tooth. See a dentist as soon as possible. It will only get worse and cause you more pain, swelling and cost.
Cavity. A whole in your tooth is likely a large cavity. Cavities are caused by bacteria that tend to have a foul odor. Food will also collect in holes and after a few hours will not smell good at all. Time to visit the dentist to have the tooth fixed before it starts to hurt or becomes infected!
Woops. Sounds like you have an infected tooth. You will probably require dental treatment with antibiotic overage. Please make appointment with a Dentist as soon as possible. .
Hole in tooth. The hole in your tooth is filled with bacteria and rotting food particles. This can cause a bad taste and smell. You didn't mention pain or swelling but this can also occur. See a dentist. It will not heal itself.