What's the Seam?

The "Seam" refers to the area where Katniss and her family live. In the poverty of District 12, the Seam is the poorest section of the district. They are the poorest of the poor.
The name originates from the predominate resource of District 12: coal. A coal seam is a dark band of coal that is visible within layers of rock. The workers in this district are charged with mining the coal, a dangerous job that claims the life of Katniss's father before the plot of this novel picks up.
The "Seam," then, refers to the region of society that is almost crushed into oblivion by external forces. Like the coal, these people are all but hidden from society, struggling to emerge from the confines of imprisonment. They, like coal, are valuable, yet their government crushes them from all sides with laws and regulations which ultimately only benefit the Capitol and its citizens.