Extraterritorial jurisdiction refers to the power of a country to make and enforce laws outside its own borders. It is like when you play a game with your friends, and you make a rule that says nobody is allowed to throw the ball outside of the yard. Even if the ball goes outside of the yard, you can still punish your friends for breaking your rule.
Now imagine that the "yard" is a country, and the "ball" is something that someone does that goes against the laws of that country. The country can still punish that person, even if they did something in a different country. That's called extraterritorial jurisdiction.
For example, let's say someone in a different country hacks into a company's computers that is based in your country. They steal information and cause damage to the company. Even though the hacker didn't physically do anything in your country, your country can still charge them with a crime and hold them accountable for their actions.
Extraterritorial jurisdiction can be complicated and controversial because it involves one country affecting the lives and actions of people in other countries. But it can also be important in protecting a country's interests and enforcing their laws.