Playlists are the minimal playing unit in Castmill. For any content that you want to display, you need to use a playlist. A playlist can be as simple as just one media item, or as complex as a layout with several sections with many items per section.
Remember that playlists needs to be programmed in a Calendar so that the player knows when to play them.
You create new playlists by clickin on the "New Playlist" button. A dialog will popup and you will be allowed to choose the layout for your playlist. You can read more about layouts here.
For a simple layout such as landscape or portrait, the playlist will have just 1 section. So you can just drag and drop your medias from the Object selection component and rearrange them as you need.
When a media object is added to a Playlist, it gets a duration that depends on the kind of object it is. If it is a video it will be the video's duration, if it is an image or a widget it will have 30 seconds duration by default. However you can easily change the duration of the item by clicking on the duration and sliding the time:
Once the items are in the playlist you can still rearrange them in a different order by just dragging and dropping the items inside the playlist:
You can always preview the playlist by using the playlist controls which are similar to a video player. If you want to move the time to a given playlist item just click on the item and the player will move the time to the start of that given item.