Purdah is the term used in the United Kingdom for the period between when a general election is called and when the results come in. The word literally means "screen", and it refers to the separation between the civil service and the elected government which comes into effect once an election is called. The purpose is to prevent the government from having an unfair advantage in the election from their incumbency. It also sets the country on auto-pilot, freeing ministers to devote themselves to the campaign.
During purdah, the government cannot announce any major new initiative or programme on which the next government might be reasonably expected to take a different view, unless delay would be detrimental to the national interest or the public finances. They also can't splash money around to try and win votes. There is also a substantial depoliticization of the civil service during this period, meaning that civil servants cannot actively defend government policy or run advertising campaigns in defence of that policy. This is especially significant nowadays because the Labour Party has constructed a vast pro-government communications structure in the civil service and turned the government into Britain's biggest advertiser.
Because British governments can choose exactly when to call an election, they have the advantage of being able to binge on vote-winning spending programmes and advertising before purdah officially begins. This usually takes the form of pumping money into regional "regeneration" projects in marginal constituencies, and is precisely the sort of thing they're not allowed to announce during purdah. This ensures a separation in time between when the government has all of its powers with which to tempt the people and when it asks the people for a new mandate, ensuring the latter have a clear head.