Apps that run on a HoloLens can contain two kinds of views, holographic views and 2D views. Apps can switch between holographic views and 2D views. This is frequently used to access system provided interfaces such as the virtual keyboard. Apps that have at least one holographic view are categorized as holographic apps. Apps that never have a holographic view are 2D apps.
A holographic view gives your app the ability to create holograms in the world around you. When an app is drawing in the holographic view, no other app is drawing at the same time -- holograms from multiple apps are not composited together. By continually adjusting the perspective from which your app renders its scene to match the user's head movements, your app can render world-locked holograms that remain at a fixed point in the real world.
The shell does not render while in a holographic view either. Any system notifications that occur while a holographic view is showing will be relayed audibly by Cortana, and the user can respond with voice input.
A 2D view appears in the shell as a virtual slate, rendered alongside the app launchers and other holograms the user has placed in their world. The user can adjust this slate to move and scale it, though it remains at a fixed resolution regardless of its size. If your app's first view is a 2D view, your 2D content will fill the same slate used to launch the app.
You can run a Universal Windows App on HoloLens that was built for other Windows 10 platforms, such as desktop or mobile. These apps are already rendering 2D views today, and their content will automatically appear on a slate in the user's world when launched.
One key use of 2D views is to show a text entry form that can make use of the system keyboard. Because the shell cannot render on top of a holographic view, the app must switch to a 2D view to show the system keyboard. Apps that want to accept text input can switch to a 2D view with a text box. While that text box has focus, the system will show the system keyboard, allowing the user to enter text.