Define the thing or list of things that should be displayed in this container. For example, in groups or popups, only a single thing will display, but for repeating groups a list of things will display.
Child elements can access the Data source from their parent. If the parent contains a list of things, the child has access to the individual list items.
A common scenario for groups is to display something, like the current user, and then have the group contain the inputs that modify the user's properties, such as text and images that display information about the user.
The Type of content determines what kind of thing you are allowed to set in the Data source field. If the type of thing does not match the Data source, it will be marked as an issue, and you must change one or the other to match.
The Data source can come from anywhere in Bubble. For example, it could be the result of a search, a custom or built-in Data source, or it could be dynamic and come from the user. The Data source will update when its source changes. For example, if the search result changes because a user modified some data used in the search constraints, the displayed list will automatically change. For additional flexibility, you can update the Data source in the Conditional Formatting Tab in the Property Editor.