What are nodes?
You'll notice that throughout your Drupal installation, and these tips, that the word "node" is used a lot.
With Content Management Systems (CMS) like Drupal, nodes are the actual content within the site. They can be pages, events, forum postings, and more.
Drupal and its modules come with several node types - web page, book page, event, forum topic, and blog included.
Advanced users can also create their own node types.
A "teaser" is a shorter version of the full node. These are often used for things like the main blog listing, lists of recently posted content, and more.
A teaser can be two different things - it could just be the first so many lines of your post, or it could be a custom synopsis of the post.
A teaser that is just a shortened version of the post is easy to do and included with Drupal. In your WYSIWYG editor, look for something that looks like this (looks like two partial rectangles with a dashed line in the middle):
If you hover over the image, it may have a hint that says something about a "page break". Put your cursor at the end of the text you want to be in the teaser. Then click this button. It will automatically set your teaser to be everything prior to that point.
A customized teaser - such as a synopsis of the post - takes additional setup by someone familiar with Drupal as this feature does not come standard. Once it is set up, you will have another text box on the page that is just for the teaser text.