Self-cannibalism is either harmless, such as chewing on the inside of your own cheek, or biting off the dead skin on the ends of your fingers; or more serious in forms of self-mutilation.
It is not uncommon for young children to chew on their fingers, or to eat blister scabs. Kids can be so gross. Some habits are carried into adulthood, nervous habits like biting fingernails. It's usually an unconscious gesture.
Like other kinds of self-mutilation, self-cannibalism involves the chewing of one's own flesh, most often on the hands and arms (the easiest parts to get to the mouth). Self-cannibalism is not very common, compared to cutting or burning.