A WordPress excerpt is basically a summary of a longer article, often used as a replacement on the blog index and archives pages to avoid needing to display the full content of each post.
For more understanding, Excerpt is an optional text associated to a Post.
Excerpts allow users to display more content in less space. It is particularly useful for content rich websites such as news or magazine sites where the website owners would want to showcase more content on the front page with links to full articles.
The default limit of excerpt is 55 words, but this can be changed.
there are three types of excerpts: Manual Excerpt which is entered into the post separately, Automated Excerpt which is generated from post content, and tag excerpt which uses the content up to the specified part where the tag is inserted.
How to display excerpts on the posts page
Go to Settings > Reading.
Scroll to For each article in a feed, show.
However, they are not required by default. If you want to create a site with consistent, good-looking, high-quality content, then make sure you have a helpful Excerpt for each post.