A deckle edge is when the pages of a book are cut in a ragged way so that they seem to be trimmed by hand. Turns out the tradition of creating those edges is pretty interesting:
The deckle edge dates back to a time when you used to need a knife to read a book. Those rough edges simulate the look of pages that have been sliced open by the reader. The printing happened on large sheets of paper which were then folded into rectangles the size of the finished pages and bound. The reader then sliced open the folds.
There’s a great article on this tradition at: