If your app uses Public with Email, Allowed Email List or Password for its sign-in settings then your users will need to sign in before they see anything in the app.
However, sometimes you may want to let users view the app first (or parts of it) and then allow them to sign in later. You can do this with the Show Sign In Action.
No configuration is needed, just add the action to a button or image component via the features settings panel.
If you want this sign-in screen to use Google sign-in, enable it in your 'Public with email' sign in settings first.