What is abnornal termination of main() function ?

Answer 1

Every termination where you don't return an int to the operating system.
When main() reaches its ending curly brace }, that's no abnormal termination because it returns 0 anyways although you didn't tell explicitly to do so.

Answer 2

Examples of abnormal termination,

When abort() is called
exception isn't catched
division by zero
etc...