Display is a particular use of type. In the days of letterpress and phototypesetting, many of the most commonly used typefaces were available in a "display face" variation.
Display fonts were created for best appearance at large "display" sizes (typically 36 points or larger) as might be used for a major headline in a newspaper or on the cover of a book. The main distinction of a display face was the lack of "ink traps", small indentations at the junctions of letter strokes. In smaller point sizes, these ink traps were intended to fill up when the letterpress was over-inked, providing some latitude in press operation while maintaining the intended appearance of the type design. At larger sizes these ink traps are not necessary, so display faces do not have them. Today's digital typefaces are most often used for offset lithography, electrophotographic printing or other processes that are not subject to the ink supply variations of letterpress, so ink traps have largely disappeared from use. This is why display cases are rarely found in the world of digital typography, whereas they were once common in letterpress printing.
When digital fonts feature a "display" variation, it is to accommodate stylistic differences that may benefit type used at larger point sizes. Unfortunately, some 20 years plus into the desktop publishing revolution, few typographers with metal foundry type experience are still working, so the misuse of the term "display typeface" as a synonym for "ornamental type" has become widespread. Read more about display fonts on Wikipedia.
