A sign is a single display of information. That might be an Announcement, Youtube video, Twitter Message, Restaurant Menu, Real Estate Listing, or anything else. For example, a real estate listing template can display realtor, pricing, pictures, and more.
A Show is like a playlist. It is a collection of signs looped together. You can assign the length of time each sign plays, and those signs will loop forever as a playlist. An example might be a school cycling through various announcements for the upcoming month.
Schedules are a collection of signs and shows that play throughout the day at times you specify. For example a restaurant might have a breatkfast, lunch and dinner menu, and wants to display those different menus and different times throughout the day.