It's an informal word for:
* A tummy: This is the most common meaning, referring to the stomach or belly.
* A car: This is less common, but it refers to a small, inexpensive car.
* A type of drum: This is even less common, but some people use it to refer to a small drum or a specific type of drum.
The specific meaning of "tum tum" depends on the context in which it's used.