Elements are the content modules that can be created within the Timeline view. The App developer chooses which Elements are available in an App. Project Settings > Elements allows you to show and hide Elements for convenience.
Elements can be configured by clicking on the Element on the Timeline or by Adding an Element in the bottom-right corner of the Timeline screen.
Changes take instant effect. If you are editing a live Element some fields can be changed post-publishing and will update instantly, some may be disabled.