A timeline is always in either view mode or edit mode.
View mode is meant for browsing, reading, or presenting a timeline. You can change timescales, units, and zoom levels, and tap on any item to view its details.
Edit mode is used to create or edit a timeline. You can drag events, images, and sections around, you can modify the values shown in the Inspector, and a tap on any item brings up a menu full of editing actions.
Example timelines always open in view mode; other timelines remember what mode you last used.
Switching Between View and Edit Modes
Switch between View and Edit modes from within the 
menu. There will also be a Viewing indicator on the top bar when in view mode (provided the screen is sufficiently wide, e.g. full screen on an iPad); you can also tap on the Viewing indicator (instead of the menu) to switch to Edit mode.