This option selects the number of characters in each group of characters in the puzzle.
For most people, leave this at "0".
This means that the resulting cipher text has spaces in the same place as the plain text input. This makes it easier to find the two letter words (for instance), and help figure out what the mapping is.
However, if you set this to something like 4 or 5, then all of the spaces are removed, and the cipher text is presented in groups of characters. Needless to say, this makes the puzzle harder to solve.