The first two or three paragraphs of your article is the "teaser". It generally serves as an introduction, hopefully drawing in readers and encouraging them to click 'Read more' to read the rest of your story. Quite frequently the only original content in a KernelTrap article is the first couple of paragraphs, followed by a relevant archived email thread from a kernel development mailing list.
Each paragraph of your article should be wrapped in <p> and </p>. This is how the CSS determines the proper formatting. For example:
<p>This is the first paragraph of my story. Lots and lots of words. Very interesting information. Useful links to other interesting information. The final sentence of my first paragraph.</p>
<p>This is the second paragraph of my story. Lots more words can be found here, as well as more interesting information and useful links. Here comes the end of my second paragraph.</p>
Do not use <br> tags to insert line breaks. By properly using <p> paragraph tags, you will end up with legible formatting.
At the end of the introduction, on it's own line add the following html comment which tells Drupal that the teaser is over:
Everything before the <--break--> will be in the introduction, showing up on the front page. Everything after it will be in the body of your ariticle. The <--break--> comment itself will not actually appear anywhere in the story.