Okay kiddo, let me explain what Hadran means when it comes to the Talmud. You know how when you read a book, you might say "I finished the book!" when you're done? Well, when people in the Jewish religion finish studying a section of the Talmud, they say "Hadran."
But it's more than just saying "we finished," it's a like a special way of saying thank you to the Talmud for teaching them all of these things. They're also promising to come back to study it again in the future, because there's always more to learn!
So basically, it's like celebrating finishing a really important book, and saying thank you for all the knowledge you gained, and promising to come back to it again someday. And even though it's in Hebrew, people all around the world say Hadran when they finish studying a part of the Talmud.