Overscan is Evil
If a gallon of gas measured 5% less than advertised, would you still call it a gallon? Many televisions have a nasty habit of hiding (literally destroying) visual information along the border of the video image, reducing the detail and clarity of the remaining visible picture that fills the screen. Overscan is the excessive scaling of video beyond the edge of a TV's screen. For example, a TV that exhibits 5% overscan fails to display the outermost 5% of the pixels along the video border.
Examples of Overscan
The following pictures show the lower right corner of a 1080p flat panel HDTV's screen, including a bit of its black bezel. This HDTV has a default overscan of about 6% and an underscan display mode that reveals every pixel of the incoming video signal. The left half of each picture shows the TV screen with its default overscan setting, and the right half shows the same portion of the TV screen and the same frame of the video but with overscan disabled - an underscanned picture. In each image, I've highlighted the details concealed by TV's default overscan mode in yellow.
Why Overscan is Bad
The relatively small percentages of video overscan I've mentioned may not sound like much, but the losses incurred scale with screen size. A 65-inch 1080p TV with 6% overscan (a typical amount) is missing 1.7 inches of video data from the left and right sides of an HD image - a total of 3.4 inches of lost visual information (horizontally.) With full-frame 1080p video, a 6% overscan equals a loss of over 124,000 video pixels - regardless of screen size. Any amount of video overscan introduced by a display device can soften the picture, and it always results in a significant loss of detail.
Manufacturers label picture size settings that reduce overscan as Just Scan, Dot-by-Dot, Screen Fit, and sometimes (accurately) Overscan with a related on/off control. Many new TVs default to a picture size setting labeled 16:9 (or Auto) that unintuitively produces an overscanned image. Also, overscan controls are often unique to each video format. So turning off overscan for 1080i/p video sources doesn't automatically turn off overscan for 720p or standard definition sources.
Fix it or upgrade
It doesn't matter if the video source is a favorite streaming app, a next-generation game console, or Blu-ray movies: a good TV doesn't waste a single pixel! You should see everything as the content creator intended! Of course, televisions can degrade image quality in many other ways, but video overscan shouldn't be one.