from Wikipedia:
168 empty chairs hand-crafted from glass, bronze, and stone represent those who lost their lives, with a name etched in the glass base of each. They sit on the site where the Murrah Building once stood. The chairs represent the empty chairs at the dinner tables of the victims’ families. The chairs are arranged in nine rows to symbolize the nine floors of the building; each person’s chair is on the row (or the floor) on which the person worked or was located when the bomb went off. The chairs are also grouped according to the blast pattern, with the most chairs nearest the most heavily damaged portion of the building. The westernmost column of five chairs represents the five people who died but were not in the Murrah Building (two in the Water Resources Board building, one in the Athenian Building, one outside near the building, and one rescuer). The 19 smaller chairs represent the children killed in the bombing. Three unborn children died along with their mothers, and they are listed on their mothers’ chairs beneath their mothers’ names.
