What are drag lines and how do i avoid them?

Im reading the rulebook for the competition im taking part in, and they said they are assessing needlwork based on absence of drag lines? What are those? I googled it but i didn't really get anything helpful. How do i avoid them? I have a photo of my dress posted a week ago or so, if anyone can give advice for how to score extra points or not get drag lines, I'd really appreciate it!