Frying pan bread is simply referring to bread cooked in a pan rather than an oven, which is essentially the same thing, maybe deeper, and sans handle. The medium itself consists of any dough recipe of the time made from available ingredients. It was extremely common in the 19th c and most examples come to us from the Civil War during which copious amounts of wheat and corn flour breads were baked in the types of cast spider skillets prevalent. It was baked in full skillet sized cakes or in smaller rolls.
The following is from the NPS living history at Manassas portraying the Wilcox's Brigade 1862 winter quarters encampment earlier this year. Dough was mixed and kneaded on a rubber blanket, then rolled into fist sized balls and loaded into a lidded skillet which was put over coals. The first run was over cooked but a second try yielded great results. The prior weekend I did the same with a cornbread cooked into the solid cake seen in some early war photography. I regretfully don't have the reenacted photo of the TX winter quarters camp of Brandy Station showing the soldier (me) holding the pan and the resulting cake.