An excerpt is a short summary of a post. It can be automatic or manual.
If you want to display a short excerpt for your posts on the blog archives, please go to Customize > Blog > Blog archives and scroll down to find the Show Excerpt toggle.
Clicking the Show Excerpt toggle will automatically cause the post excerpt to display. At the same time, you’ll notice two options show up Excerpt Length and Read more link. The Excerpt Length option controls the number of words that show up in your excerpt. Enabling the Read more link adds a link to each of your posts that point to the single view for that respective post.