Celts were a group of people who lived a really, really long time ago in Europe. They had their own special language, art, and culture. Nowadays, there are still some people who say they are "modern Celts" because they feel a special connection to the old Celts and want to keep their traditions alive. These modern Celts might speak a version of the old Celtic language, wear clothes that were popular a long time ago, and study old Celtic stories and music. It's kind of like when you dress up in your favorite superhero costume and pretend to be that superhero - these modern Celts are dressing up like the ancient Celts and pretending to be like them. It's really cool that they care so much about their history and want to keep it alive!