On Deck is a unique feature introduced by Plex to make quick access to your favorite media items simple. It's a quick-access bar where Plex apps show you items that have recently been added to your collection and those that you're part-way watching. On Deck updates dynamically as you watch items from your collection. Different apps may visually present these lists differently, but the concept is consistent.
If you have movies or TV episodes that are recently in-progress, they'll appear in the Continue Watching area for super-quick access. If you haven't played the item for about a week, it will no longer show up in Continue Watching, but your progress is still remembered.
Together, think of these as trying to provide you with the answer to the question, "What do I want to watch?"
Continue watching displays movies that have been started, but not finished–so they're "in progress".
Recently Added displays movies that have been added to the library recently.
On Deck for TV libraries is where things really start to get smart. If you stop watching in the middle of an episode, then it will appear here because it's "in progress". If you recently completed watching season 3 episode 17 of a series, then On Deck is going to have season 3 episode 18 listed for you since that's the next episode in line and it's pretty likely that you might be interested in watching it. The content is also ordered based on what you most recently accessed.
Continue watching displays episodes that have been started, but not finished–so they're still "in progress"
Recently Added displays episodes or seasons that have been added to the library recently.