Sindarin is a made-up language that people use for fun or as a way to express themselves creatively. Just like when you play make-believe with your friends and make up your own secret language, people who like the Lord of the Rings books and movies have made up their own language too!
Sindarin is based on an imaginary world created by a man named J.R.R. Tolkien. He made up languages, cultures, and creatures to help build the world of his stories. Some people who really love his books and movie adaptations have gone the extra mile to learn and speak Sindarin to make their personal experience with the story even more special.
It's kind of like being part of a secret club where only people who know the language can communicate with each other. Just like how you and your friends might have a special way of talking that only you can understand.
So, in summary, Sindarin is a made-up language used by people who love the Lord of the Rings books and movies to express themselves creatively and feel more connected to the imaginary world of the story.