The Brittonic languages are a group of languages that were spoken a long time ago by people who lived in Britain. They were part of the Celtic language family, which also includes languages like Irish and Scottish Gaelic.
Each Brittonic language was spoken in a different part of Britain. For example, Cornish was spoken in Cornwall, Welsh was spoken in Wales, and Breton was spoken in Brittany (a region in France that used to be part of Britain).
These languages had a lot of things in common, like grammar rules and vocabulary, but they also had some differences. Think of it like how you and your friends might all speak English, but you might use different words or have different accents depending on where you're from.
Over time, the Brittonic languages started to change and evolve. Some of them, like Cornish, eventually died out and are no longer spoken. Others, like Welsh, are still spoken by people today.