A shotgun house is a type of house that is usually very long and skinny. Imagine lining up a bunch of boxes next to one another like a train. That's kind of what a shotgun house looks like from the outside.
When you walk inside, the rooms are usually arranged one after the other without any hallways. Imagine a bunch of doors lined up on one side of the house. When you open the first door, you're in the living room. Then you walk through the living room to get to the next door, which leads to the dining room. You keep walking straight through the dining room, and there's another doorway that leads to the kitchen. There might be a bathroom or bedroom at the very back of the house.
The name "shotgun" comes from the fact that if you opened all the doors from one end of the house to the other, you could shoot a shotgun and the bullet would go straight through without hitting any walls.
People started building shotgun houses a long time ago, because they are cheap to build and don't take up much space. They are still popular in some parts of the world today, especially in places like New Orleans.