I think you might be talking about the bit that separates the teaser at the top of your story, which is displayed on the front page, from the rest of it.

There's a button above the edit text box, second from the right, which is the one you need. It has an 'A' above a dotted line and a 'B' below. This actually inserts the string "<!--break-->" into the text. This string is considered to be a comment by the source processing software and is ignored, so it would be safe for you to add that into your own text file if you wished.

Penny