When creating a post or a page it is best to use mostly relevant keywords in the first 139 characters of a post. The is what is shown by Google on the search results page. You want to make sure the post is summarized effectively in this first 139 characters. It is actually 159 but other meta data may use some of these characters so it is best to keep it to 139 or 140 characters.
One method use to create SERPS as they’re called is make a list of keywords having to do with the post, then make a paragraph using as many of those keywords a possible. Make sure you do not use to many if’s, and’s, but’s, the’s, is’s and’s and a’s as possible for these add nothing to the SERP and they use characters. An example of a SERP is: A SERP is a 139 character summary of your post that’s filled with many relevant keywords. The SERP is what is viewed in the search results page.
If you fell to create a good SERP in the first 139 WordPress gives you a second opportunity with the “Excerpt” box that is below the post area.
Doing this is part of SEO (Search Engine Optimization). In other words how does Google perceive you content.