Let's say you don't know the exact time when dessert will be served, but you do know it'll happen between the entrees and first dance.
Here's how to mark the item as TBD:
Click on the time of the item you wish to mark TBD.
Choose "Keep Inline" if you want the item to stay in its slot in the timeline, or "Move to Top" if you want to keep all your TBD items in one place.
Feel free to drag and drop the item anywhere in the timeline that you want it to go.
Here's a simple tutorial to walk you through the process: