This is the episode description, sometimes called episode notes or show notes. It's what people will see when they play the episode in their podcast app, and on the web. You can do some fancy things with your episode notes, like embed videos from YouTube or photos from Instagram.
Your episode notes automatically adjust depending on the app the listener is using.
We use the Markdown syntax for episode notes, which is a way of writing formatted text in a way that humans and machines can read easily. Find out more about Markdown at https://pcast.link/kbmarkdown/