A podcast description is a paragraph that defines your podcast. This is the first thing that a user reads when they land in your podcast to understand the type of content your podcast covers. This is like a book summary. It is a great place to mention your social handles, website, or your email address for listeners to contact you.
Think about your description as the keywords through which your listeners will find you (think about it like podcast SEO and you'll be good to go). Add words that you think people may write when searching for a podcast like yours.
An episode description is a summary of your episode. This gives the listener a brief on what the episode is all about and what you will be talking about in the episode.