Excerpts can be really handy in allowing you to craft a short summary of your post. The WordPress codex has this to say about them:
“The WordPress Excerpt is an optional summary or description of a post; in short, a post summary.
The Excerpt has two main uses:
It replaces the full content in RSS feeds when the option to display summaries is selected in Dashboard › Settings › Reading.
Depending on the WordPress theme, it can be displayed in places where quick summaries are preferable to full content:
I tend to use them on widgets which display featured posts, so I can make them as enticing as possible to encourage visitors to explore the site more.
And do you know where to find your excerpt? There should be an excerpt box underneath the editing section on each page or post. If it’s not there, check the screen options at the top of the page and make sure the Excerpt box is ticked.