Placeholders are temporary activity plans on the PLANNS screen that you generate from within the GUESTS screen.
Use them as a quick addition to the Planns screen and return later to fill in the rest of the details. That way you'll see them on the Planns page and you won't overlook them.
More about Placeholders here https://intercom.help/planniac/en/articles/8665315-placeholders#h_20562df328