A slate is a thin window style control for displaying 2D content. For example, you can use it to display plain text, images, or documents. It offers a grabbable title bar with buttons for 'Follow me (tag-along)' and close functionality. The content within a slate can be scrolled via direct manipulation just like a physical touch screen. You can use gaze, air-tap and drag gesture or a motion controller's pointer as well.
Slate in MRTK (Mixed Reality Toolkit) for Unity