underground railroad quilts were the quilts that women made during times of slavery and the opposition to it to help the black people find their way to freedom in the north house by house. There are quite a few quiltblock designs that would mean a certain thing to the slaves traveling north and the safest way (at the time) was to put them into a quilt that could be hung on a porch or fence to guide the slaves. Google "History of the Underground Railroad" and "Underground Railroad Quilts" to learn more.
It is totally fascinating how women find a way to communicate and help others in a male dominated time in our history. If the schools had taught this kind of stuff in history, I would have paid a lot more attention.