In Great Britain, members of Parliament are not allowed to resign. This rule was declared in 1623 at a time when Parliament was not as influential and service in Parliament was sometimes seen as a resented and onerous duty rather than a position of power and honor. However, there is another rule which states that if a member of Parliament accepts an "office of profit" from the Crown, he or she is obliged to leave his post so as not to be unduly influenced by being in the pay of the King. Thus when a member of the House of Commons wishes to resign for some reason, they simply apply for an "office of profit under the Crown" and are thus "forced" to give up their membership in Parliament.
Steward of the Chiltern Hundreds (or more officially, "Steward or Bailiff of Her Majesty's Three Chiltern Hundreds of Stoke, Desborough and Burnham in the county of Buckingham") is one of two offices used today to effectively allow MPs to resign. The other is the Steward of the Manor of Northstead.
In practice, the application for the Stewardship is always accepted and approved, to allow MPs a resignation option. To maintain the legal fiction of the office, the Stewards are actually paid a nominal amount of money to make them "paid officers of the Crown," and continue to hold the office until another member of Parliament wants to resign, which sometimes takes years.
In the past, numerous other minor crown offices were used by MPs to resign in this manner, but today only Steward of the Chiltern Hundreds and Steward of the Manor of Northstead are used in this way. The two positions are used alternately, allowing up to two MPs to resign at the same time. If more than two MPs resign at once, as when 15 Ulster Unionist MPs resigned en masse in protest of the Anglo-Irish Agreement on December 17, 1985, the resignations are theoretically not simultaneous but instead are spread out throughout the day with each resignee holding office for only a few minutes.
In the Middle Ages, the Chiltern Hundreds were a wild range of heavily wooded hills that were a notorious hideout for outlaws and other unsavory types (a "hundred" was a traditional subdivision of an English county). Accordingly, a Crown Steward was appointed to maintain law and order in the area. However, over time the region became more civilized, and the stewards ceased to be required by the 16th century. The office remained on the books, but generally went unfilled after the 17th century.
In 1751, however, member of Parliament John Pitt, who wished to vacate his seat for Wareham and instead stand for Dorchester, had the clever idea of applying for the Stewardship and thus effectively resigning from his seat in Parliament. His plan worked, and a tradition was born.