In the editor, the calendar symbol lets you choose when your post should appear. Until then only you see it.
In the same way you decide when a post disappears by itself, for example an announcement that only holds until a date.
Changing or cancelling
You change or cancel the time in the post’s menu (⋯) on your profile. If you cancel the time of a scheduled post, it appears at once. If it is still waiting for its video, it appears as soon as the video is ready.