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Great question: To get a direct answer to your question, I'll refer you to an article written about SEO by the Medium creators. They have done a fantastic job of describing the SEO description function. Here is the link:
My take on SEO Description: The SEO description's default option seems to be the first 150-200 words of your story. Of course, you can customize your story's SEO description to be any text. Personally, I try to make my first 200 words as compelling as possible to invite readers to explore further. However, you may want to include your SEO keyword in your title + first 200 words (introduction) + SEO description. This is critical if you're going to get ranked on Google and other search engines.
Hopefully, my answer is not too long, and it answers your question.