Sheets provide a way of structuring your ideas and purposes for your app. When you create an empty app, it is good practice to first build a structure of empty sheets, where each sheet represents an idea or a goal. This will give you, and others who you will share your app with, a good overview of the app.
For example, let us say that you want an overview of your business's key metrics, view sales and margins by state, region and product, and also sales by city and salesperson. Instead of having all this information in one place, you could structure it by having one sheet for each of these purposes.
A sheet is where charts and tables for data visualization are placed. An app can include one or several sheets.
The selections that you make affect visualizations regardless of which sheets they are located on.