The term "Noble hops" historically refers to 4 varieties of hops (specifically Humulus Lupulus) grown primarily in central Europe, especially Germany and Czechoslovakia. They are relatively low in alpha acid (AA) bitterness compared to many other hops cultivars. The four Noble hops are 'Hallertauer Mittelfrueh' or 'Hallertau', 'Tettnanger' or 'Tettnang', 'Spalter' or 'Spalt', and 'Saaz.' They are each named for a specific region or city in which they were first grown or primarily grown.
Their low relative bitterness but strong aroma are often distinguishing characteristics of European-style lager beer, such as Pilsener, Dunkel, and Oktoberfest/Marzen. In beer, they are considered aroma hops (as opposed to bittering hops); see Pilsner Urquell as a classic example of the style, which showcases Noble hops.
The land where the hops were grown affects the hops' characteristics, similar to grapes. Much like champagne may only be called "champagne" if the grapes were grown in the Champagne area of France, Noble hops may only officially be considered Noble if they were grown in the areas for which the hops varieties were named. An equivalent style of beer is lambic, which may only be officially called lambic if brewed in a specific region of Belgium.