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 ...
