When you write your Post Title, the Permalink is generated automatically using the words in the title. However, your Post Title may be long, or may not be using the keywords needed for SEO. You want to make the URL for your post concise, informative, and include key words to help search engines categorize your post.
Here's an example of a long post title where the Permalink needs to be edited from the default:
My Extremely Fun Time Swimming with Sharks in the Bahamas
The default Permalink would be:
A better Permalink would focus on the main topic of the post which is swimming with sharks in the Bahamas, and you could shorten the Permalink to:
This URL explains what the post is about and includes the keywords - swim, sharks, Bahamas - but isn't too long.
You can edit the Permalink any time before your post goes live. It is important to not change the Permalink after you publish the post as this will break any links which have already been shared.
When composing your post, click the blue Edit button below the Post Title field, update the Permalink, and click Done. Make sure to use dashes instead of spaces to separate keywords in the Permalink.