On Deck is a unique feature introduced by Plex to make quick access to your favorite shows a breeze. It's a quick-access bar where Plex Apps show you Movies that have recently been added to your collection and those that you're part-way watching. On Deck updates dynamically as you watch shows from your collection. On Deck comes in two flavors, one for Movies and one for TV Shows. Different Apps present these lists visually differently but the concept is consistent.
The Recently Added list for Movies shows you the media most recently added to your collection. Watching a Movie to completion will remove it from the the Recently Added list completely. Partially watching a Movie will move it from Recently Added to On Deck where you can quickly take-up where you left it off. In the example below, partly watching "The Fly" moves it from Recently Added to the first position of On Deck.
On Deck is a list of Movies that you're part way through watching. If you always watch your movies to the end, the list will be empty. If you choose a Movie from the normal list (i.e. not from the On Deck bar) and partly watch it, it will appear in the On Deck list.
Different Apps show On Deck slightly differently from each other, depending on what works best for the platform. Some show a Unified On Deck which mixes Movie and TV Show contents. Others - like Plex for Home Theater - separate On deck for Movies and TV Shows into their own bar. In the example below (Plex App for iOS) notice that both On Deck and Recently Added have both Movies and TV Shows mixed together.
Whilst Plex for Home Theater has a separate On Deck for Movies and TV Shows.