Content displayed in a feed, such as a list of Articles, usually displays a summary blurb. Drupal will show the first sentence or two from your content (approximately 250 characters) by default and many times this is sufficient. However, should you want to control exactly what appears in the summary, you have two options:
Use the "Insert Teaser Break" Button
Use this option when you want your teaser to display the exact same text as your content, but end in a specific place. This button, indicated by a dashed line, will insert a divider into your content and everything above the line will appear in your summary blurb.
Use the "Edit Summary" Option
Use this option when you want summary text that differs from the first sentence of your content. Click the "Edit Summary" link (next to the word "Body") and type in your desired text.