There are three episode types available: "full", "trailer" and "bonus".
Full is the default value and stands for a regular episode of your show.
A podcast can add a trailer episode. It will be shown on the podcast page, for example on Apple Podcasts. It is like a teaser of your podcast. You can use it to present your podcast to a potential listener and make him or her subscribe to your show.
A bonus episode adds content that would normally not fit into your regular episodes. As an example, it could contain a "making off" your show.