Whether you explicitly created it or not, you have a conceptual model of your app, or site. What parts do what. What they are called. And how they fit together.
This is different from the visual model, for your iPad app could look very different from your iPhone app and your web app.
So it’s important that your users grok the app the way you intend them to, and the best way is to quickly and simply walk them through it at first use.
Things 3 does it quite well for a sophisticated app:
Note that doing this is straightforward because Things has also designed its main, home screen to reflect that conceptual model.
It could have been a set of menu options tucked away under s hamburger, or tab bars with a ‘more’ tab for expansion. Or worse the model could have been multi-level, making it truly difficult to communicate. See Swarm’s splash screens attempting to do the same thing:
It’s a great app that I like using, but the screenshots here don’t do nearly as good a job setting expectations with a new user about what the app’s about (Swarm recently radically redesigned their app, so even an existing user was, for this purpose, a new one).
Things designed their app so that the model is (mostly) single-level, and reflected on a single screen.