Duration refers to how long a buff, debuff, or spell-like effect lasts (total) or how much time is left on the buff. The shortest durations last only a second or two while the longest can last up to 2 hours. This is the general type of duration as opposed to the duration of items.
Duration time display can be toggled on and off and (when on) causes the actual time left to display under the buff or debuff icon via the built-in Interface settings. This applies to both buffs and debuffs.