Atgeir is like a very fancy stick that some people used to use a long, long time ago. It was sort of like a spear, but instead of being pointed like a needle, it had a big, flat, angled blade at the end.
Atgeirs were used by people who lived in an area that is now known as Norway. These people were called Vikings, and they were really good at sailing on boats and fighting battles. When they were in a battle, they would use their atgeirs to try to hit the other side's soldiers from a distance, before the enemy could get too close.
The atgeir was a very special weapon because it was really long and could poke the bad guys from far away. It was also very fancy because the Vikings would decorate it with pretty carvings and other decorations. Sometimes they would even put gold or silver on it to make it look extra special.
Nowadays, people don't use atgeirs anymore because we have other, better weapons to use. But we can still look at them in museums and pictures to learn about the Vikings and how they used to fight.