When you upload or edit an episode you can select the episode type in the details section. There are three types of episode.
Full episodes are standard episodes. You'll use this type most of the time.
A great way to let new listeners get an idea of what your podcast is about before they subscribe. You may wish to publish a promo to announce coming episodes or seasons with this episode type. A trailer is often featured in Apple Podcasts and other apps.
A bonus is extra material released outside of the running order of your episodes.