Meta Title and Meta Description
The Meta Title takes the form Post Title Separator Site name by default (this can be changed in SEO Settings. Separator is a character such as Dash (-) or Pipe (|).
Meta Description is the start of the blog post content, truncated at 160 characters or less.
While ok for length, this is not usually ideal and doesn't read well.
Although you can replace it with your own typed one, I like to kill two birds with one stone here and create an excerpt. This is useful to have as many themes display the excerpt in post summaries.
It's on the right-hand side.
Write your own excerpt. You are summarizing the essence of this post in 160 characters or less.
As soon as you create your excerpt and update your post, it's automatically used as the meta description.
I don't understand why you have to choose a "Post Excerpt" in the meta description. I myself do NOT use tags in the meta description and I put my own text in it (different from "Post Content") and no longer than 160 characters... Is that a problem?
Since every post or page gets its own unique meta description, can't it just be typed in? Or am I missing something?
Best regards,
Have a blessed weekend.
Monica