The Romans tried very hard to conquer Britain. Britain had tin mines and the Romans needed tin. The Romans called the people living in Britain barbarians. We refer to these people today as the ancient Celts.
The Romans never quite succeeded in conquering all of Britain. The Roman Emperor Hadrian ordered a wall to be built in Britain, to clearly mark where the Roman territory in Britain ended and the barbarian territory began. This wall was called "Hadrian's Wall". It wasn't really a wall across Britain. It was more like a small stone fence with guard posts along it. Parts of the wall still exist today.