What is the meaning of this long line in this sentense?

Solution 1:

Such omissions are a stylistic consideration, and as such, they are dictated by the governing manual of style. The Chicago Manual of Style recommends one of the following:

• 2-em¹ dash, to indicate missing letters. Thus if the missing place was Berkshire, one could write ————shire. (You'll have to pretend that the length of each of the dashes is 2-em. The HTML entity set contains such a character, but it doesn't render in my browser.) Opinion differs on the spacing between 2-em dashes. Some recommend no spacing (so you'd see a solid horizontal line); some recommend a hair space² (so you'd be able to count the missing letters).
• 3-em dash, to indicate a whole word omitted. To disguise the letter count, the publisher might have considered Berk such a word and rendered the place as ———shire. (Again, you'll have to pretend that the three dashes form a single horizontal line of length 3-em, since my browser won't render the 3-em dash entity.)

———

1. An em is a typographic measure equal to the point size of the font. Historically, this was the width of a capital M in a font.
2. A hair space is a narrow spacing character, shorter than a thin space, which is one-fifth of an em.

Solution 2:

I think the point in this case was to avoid offending any particular real-life officer. Mr. Wickham is a very disreputable character, and there were probably a number of disreputable characters among any reasonably sized group of real-life officers. So if she had named Cheshire, Berkshire, Shropshire, Yorkshire, or some other county, there might have been speculation that Mr. Wickham was modeled after some particular officer stationed there.

So she used ———shire.