The Video Safe Frame provides a guide to help avoid rendering portions of your image that might be blocked in the final output.
For example, to ensure that no unintentional black areas are visible on a television screen, broadcasters intentionally "overscan" the video image. The result is that portions of an image around the edges are not visible on a typical set. In addition, portions of images on 35mm slides can be partially covered by the slide mounts.
You can adjust the size of the safe frame as a percentage of the outer display rectangle with the Safe-Frame controls in the Viewport Configuration dialog. Depending upon its size, the safe frame can be used as the equivalent of a "title safe frame" (the area inside which it is safe to display titles), or an "action safe frame" (the area inside which action may occur without significant loss of information).
To view the frames, choose Show Safe Frame from the viewport right-click menu (displayed when you right-click the viewport label). Three rectangles, one yellow, one green, and one pale blue, appear in the camera viewport. The outer, yellow video rectangle is the area and aspect ratio of your current display. The middle, green rectangle is the action safe zone. The inner, pale blue rectangle is the title safe zone.