I got a hole in my teeth. is this what a cavity is?

Could be...: Or it could be a space left from a filling that fell out, or it could be result of an erosive agent (acidic fruit juice or soda pop, etc.).

Basically, yes: The term cavity refers to the damage that a tooth incurs from an oxidative process. Essentially, it is similar to rust on metal. There are many degrees on which to judge the level of a cavity. But once present, it will not go away. And over a period of time, it can only evolve into a much worse situation. By the time a hole forms, it is an advanced situation that requires immediate attention.