The phrase "building forts" means creating a small, makeshift shelter or play area using blankets, pillows, or other materials for fun or imaginative play. Full definition
It was included in the Adams - Onís Treaty of 1819, along with the rest of the Florida peninsula, and the U.S. soon after built a fort in what is now downtown Tampa. (valuepenguin.com)
If you are stuck inside on a rainy day, let your kids build a fort with what they can find in the house — couch cushions, chairs, blankets, and more. (verywellfamily.com)
The one about building a fort out of sheets is totally my kids. (littletechgirl.com)