When composing a new post using Publish, you are given the option to write a Summary in step 2 of the wizard.
The summary does not appear in the actual post. Instead, it will be displayed in news listings in your newsroom, in search engine results and when shared in social media.
Try to keep it short, so users have an incentive to read more :)
If you wish to display the summary in the actual post as well, just copy it to the top of your message.