The Mannerheim Line was like a big wall built by Finland to protect itself from Russia. Think of it like building a really strong fence around your garden to keep animals out.
This wall was named after a Finnish guy named Field Marshal Mannerheim who was very famous and a hero in Finland. He was the person who came up with the idea and helped build it.
The wall was made of concrete, steel, and lots of other materials that were really tough. There were also lots of bunkers and tunnels, kind of like secret hiding places inside the wall, just in case there was an attack.
The Mannerheim Line was built in the 1930s and it was really important to Finland because they had a hard time with Russia in the past. Finland wanted to protect its people from any harm, so they made sure to build a really strong wall to keep their soldiers and people safe.
In 1939, Russia and Finland started a war, and the Mannerheim Line was really helpful in keeping the Finnish people safe. It helped to slow down the Russian army and also gave the Finnish army a better chance to fight back.
So the Mannerheim Line was like a big strong fence that helped Finland feel safe and protected from Russia.