After you add your content to the text block of your “write new post” page, scroll down to the “optional excerpt” box.
Fill in this “optional excerpt” with a one-sentence summary of your post, using your post's main keyword as close to the beginning of the sentence as possible.
This sentence shows in your RSS feed as the description / summary for your post.
So consider writing it in a “teaser” style that makes people curious enough to click the article title in your feed so they can read the full post. For example, make it fascinating, funny, shocking, relate it to a current hot event in your niche, or even leave it as an unfinished sentence.