Hi Radimir
andronopoulos is correct, I will try and explain in more detail:
The ‘red line’ is the ‘Read More’ line. This governs where the ‘Read More’ button will appear in the various categories, like this:
This article looks like this in the editor:
As you can see, I positioned the ‘Read More’ red line to make the ‘Read More’ article look correct in the Category.
You insert the ‘Read More’ red line by clicking the ‘Read More’ button just underneath the main window in the text editor.
If you see a ‘Grey Line’ this is a ‘Page Break’ which means the article will have more than one page, and this is positioned where you need the article to split from page one to page two.
Hope that helps!