The slug that is produced when you publish a post defines what the permalink will look like.
Example: Blog url: myblog.wordpress.com, post title/slug: “My First Blog”
The permalink ends up like this:
EDIT: and yes what “helpdesk” said.
If the post title is very long (and even longer for the trackback url) it results in a very long permalink so the slug can be changed for brevity. Just be sure the slug is in the same format: no capital letters and words-separated-like-this.