A Unity canvas is a two-dimensional planar surface that is a container for UI graphics, such as menus, toolbars, and information panels. In conventional applications, canvases are commonly rendered in screen space that overlays the scene's gameplay graphics and has the ability to stretch and conform to a huge variety of screen sizes, aspect ratios, and orientations (landscape versus portrait). In contrast, in VR, we never use screen space because the VR "screen" has no edges and differs for the left and right eyes. Instead, in VR, we use a world space canvas that floats (albeit still on a two-dimensional surface) in the same three-dimensional space as all your other Scene objects.
Unity's UI canvas provides many options and parameters to accommodate the kinds of graphical layout flexibility that we have come to expect not only in games but also from websites and mobile apps. With this flexibility comes additional complexity...