Basically its the layer of fat just under the skin of the pig, the skin is still attached !! No meat on it !! Usually salt cured. Certainly not the healthiest of foods/snacks but once fried, it is very crispy and tasty.
I think that cracklins are made from fatback ! And eggs fried in fat back renderings are most excellent !!
And the fatback grease (or bacon grease or sausage grease) mixed into the grits. Heaven!
BTW, cracklins should be what’s left after rendering lard.