An extraterritorial operation is like playing in your neighbor's yard without their permission. Just like how you have a place where you live, countries also have their own places called territories. The word "extra" means it's not in their own territory - it's somewhere else.
When a country sends their soldiers, spies or any other people to another country to do something, like spy on people or stop bad guys, it's called an extraterritorial operation.
It's like when you visit your friend's house on the other side of town. You're not allowed to do anything bad or break any of their rules just because you're not in your own house. It's important to respect the rules and boundaries of the place you're visiting, just like countries need to follow the laws and rules of the place they visit during an extraterritorial operation.