What is it?
An excerpt in wordpress is just a tiny part of your post, shown rather than the full post with a little Read More link to go to the full article.
How to only display an Excerpt of your post. There are two main ways to do this.
In your posts click the More button at the point where you want the excerpt to end and a more tag will appear when the post is published.
In your index.php (in most themes) change the section in the loop from the_content to the_excerpt.
As always, if you are changing code, back up the file you are using first and remember that if your theme is updated you will lose the changes.