Screen Space-Overlay

Screen Space-Overlay is the default rendering mode. When you think of UI, usually the first picture of UI that pops in your head is the type that would be rendered in Screen Space-Overlay. This rendering mode overlays all the UI elements within the Canvas in front of everything in the scene. So, UI items like heads-up-displays (HUDs) and pop-up windows that appear on the same plane and the screen, will be contained within a Screen Space-Overlay Canvas.

Remember that when a Canvas is using the Screen Space-Overlay render mode, you cannot adjust the Rect Transform component of the Canvas. This is because the canvas will automatically resize based on the size of the screen (not the camera):

When you have Screen Space-Overlay selected, the following properties become available:

• Pixel Perfect: This option can make the UI elements appear sharper and less blurry. This can cause performance issues, so only use it if absolutely necessary.
• Sort Order: This option applies only when you have multiple canvases with Screen Space-Overlay render modes. The higher the number, the closer to the items within the canvas will appear to the person viewing the scene. In other words, higher-numbered canvases will appear on top of lower-numbered canvases.
• Target Display: If you are creating a PC, Mac, Linux Standalone game, you can have different cameras display on up to eight different monitors. You can also have different UI for each monitor and also where you will tell the Canvas which of the displays it will render on.