It describes the state in which Parliament is dominated by the government of the day. It refers to the fact that the legislative programme of Parliament is determined by the government, and government bills virtually always pass the House of Commons because of the nature of the governing party's majority.
Parliament consists of the House of Lords, the House of Commons and the Monarch. The customary common law rule is that in order for a bill to become an act of Parliament, it is necessary for it to be passed in both the Commons and the Lords. The bill will then go before the Monarch who has formal discretion whether to assent to the bill. On receiving Royal Assent, it will become an Act of Parliament and will be applied by the courts.
Such is the theory; in practice Royal Assent has become a formality, the monarch has not refused (or threatened to refuse) assent to a bill for some 300 years (Queen Anne in 1708). Further, since 1911, the House of Lords has lost its position of equality with the Commons. The Parliament Acts of 1911 and 1949 reduced the power of the Lords from an absolute veto to a suspensive veto. Once the same bill has been passed by the Commons and rejected by the Lords in two different sessions of Parliament, a third introduction of the bill will require only the consent of the Commons. Such a bill will then go for Royal Assent and will become law, irrespective of the view of the Lords. The Commons have, therefore, become the dominant component of Parliament - whoever controls the Commons controls Parliament, the primary legislative body of the land.
The government, so long as they can keep their MPs on-side, stand an excellent chance of getting their legislation through the Commons. The Lords may or may not also approve the legislation, however a combination of judicious compromise from the government, combined with the Salisbury Convention and the overarching threat of the Parliament Act means that most legislation also manages to get through the Lords. Royal Assent then invariably follows.
Hailsham coined the expression 'elective dictatorship' to describe this situation in which control of the Commons (and thus of Parliament) by the government is actually weak. His paper was published as a criticism of the Labour government of Harold Wilson and James Callaghan. He saw these weak governments as undemocratic, as despite their slim hold on the Commons they were able to pass a large number of their bills. He saw this as undemocratic as they didn't reflect, as Hailsham saw it, wide enough support in the country. Many have interpreted Hailsham's criticism as being one against large majorities. In fact, he actually saw these as more democratic, as they had commanded more support at elections.
Some groups, such as Charter 88, have argued that a codified, written constitution with appropriate checks and balances is also essential to solving the problem of executive dominance, although again without popular success.