Long-arm jurisdiction is like when your mom tells you to clean up your toys even though she's in another room.
Basically, it's when a court has the power to make someone from another state or country come and appear in court because that person did something that affected someone in that court's state or country.
For example, if someone from New York did something to hurt someone in California, the California court might be able to make the New Yorker come to California to face the consequences of their actions. This is like your mom making you clean up your toys from another room - the court has the power to reach out and make the other person come to them.
This kind of jurisdiction is important because it makes sure that people are held accountable for their actions, no matter where they are.