Hadrian's Wall was a big wall built a really, really long time ago by the Romans. The wall was built in northern England to help keep the Roman Empire safe from attacks by barbarians who lived to the north.
The wall was first built in the year 122 AD, and it stretched all the way across the country from the east coast to the west coast. It was about 73 miles long, which is like walking from the future King's Cross Station in London all the way to Luton Airport.
The wall was made of stones that were piled up high, and it was really thick, so it would have been really difficult for anyone to climb over. There were also big towers along the wall where soldiers could stand guard and watch for any enemies who might try to sneak past.
The wall was named after the Emperor Hadrian who ordered it to be built. He was the most powerful person in the Roman Empire, and he wanted to make sure that his empire was safe from harm.
The wall was a really important thing for the Romans, but eventually, the empire fell apart, and the wall wasn't needed anymore. Over time, parts of the wall were taken down, and stones were reused to build other things. But some parts of the wall are still standing today, and people can go and visit them to see what it was like a really long time ago.