Currer Bell is refferring to critics who would base their criticisms upon the fact that she breaks so many societal 'walls' in the book, particularly those having to do with the roles of women. Bell explains that the conventional 'rules' are not necessarily 'correct', and to break those rules should not be seen as a big deal. This was an easy way to justify the content of Jane Eyre, especially since it was written by a woman. By Currer giving this disclaimer, Bronte was a little bit safer from ridicule.