In video, letter-boxed is a term used to describe an image that does not fill the frame vertically.
This is commonly seen when widescreen films are shown on TV where you see black bars at the top and bottom. This is because the film was shot in a wider format than your screen and your TV fills the frame horizontally.
The term Pillar-Boxed is the opposite of this where, commonly seen when older 4:3 content is shown on a widescreen that fills the frame vertically but not horizontally.
Our staff will always ask what format your content is before an event to avoid this but if we are presented with content that would become letter-boxed or pillar boxed on the screens we have provided we can scale the image to fit both horizontally and vertically. The downside to the is that the image is either cropped and information lost or the image is stretched to fit.