ED stands for Extra low Dispersion glass. With this more expensive option you will find that all colours come to exactly the same point of focus. With binoculars and telescopes without ED glass you may notice false colour around the edges of an object (chromatic aberration). With ED glass you will not get this.
As a result, Optics with ED glass will be brighter, sharper and with truer colours.
With some optics the ED glass is of a higher manufacturing quality, or may have an extra element in the construction. Obviously with these premium binoculars and scopes you will pay a premium price. Many (including myself) will argue its money well spent.