Okay kiddo, celtic toponymy is all about the names of places in Celtic languages. Just like you have a name, places like cities, rivers, and mountains have names too.
Celtic toponymy is special because it's about the names given to places by people who spoke Celtic languages a long, long time ago. We can still see some of these Celtic names in modern names of places. For example, a big city in Scotland is called Edinburgh, but in Scottish Gaelic, it's called Dùn Èideann.
The names that we use for places can tell us a lot about the history and culture of the people who named them. Sometimes, the names come from stories or legends. For example, a place called "The Fairy Glen" might have gotten that name because people believed that fairies lived there.
Overall, celtic toponymy is all about learning about the history and culture of the people who lived in places before us by looking at the names they gave those places.