Screenwriting is like writing a story that you're going to watch on TV or in a movie. Just like when you read a book, a screenwriter writes down all the things the characters say and do.
But because it's a TV show or movie, the screenwriter has to be really specific about how everything will look on the screen. They have to think about what the characters will wear and where they'll be sitting or standing. They also have to think about how the camera will show everything and what special effects might be needed.
The story is usually split into different parts, called scenes. Each scene is important and helps move the story forward. When the screenwriter is finished writing, they give their script to a director who will then create the TV show or movie.
So, imagine you're watching an episode of your favorite TV show. All the things the characters say and do were written down by a screenwriter before the show was even made!