A policy rate is the interest rate a central bank sets to steer borrowing and saving in its currency. Commercial banks' own rates on mortgages, loans and savings tend to follow it, which is how a committee decision in London or Washington ends up in your monthly payment.
Each central bank names and runs its rate in its own way. The Bank of England's is Bank Rate, which it pays to banks holding money with it and charges on its loans to them; the Monetary Policy Committee sets it at eight meetings a year. The European Central Bank publishes three key rates, for its deposit facility, main refinancing operations and marginal lending facility, and they apply alike in Germany, France, Spain, the Netherlands, Ireland and Finland, where the Banco de España speaks of tipos de interés oficiales and the Bank of Finland of the ohjauskorko. The US Federal Reserve sets a target range for the federal funds rate rather than a single figure, and Sweden's Riksbank calls its rate the styrränta.
The purpose is to keep inflation near a target: in the UK the government sets the Bank a target of 2%, as the Bank of England explains. Raising the rate makes borrowing dearer and saving better paid, which slows spending and, after a delay, price rises; cutting it does the opposite.
The effect on you depends on your deal. A tracker or variable mortgage moves when the rate does, while a fixed rate stays put until the fixed period ends. The site keeps the current level and every change in its rate trackers, starting with Bank of England Bank Rate, with the others listed under /data. When you plan a mortgage or a savings target, try a rate a point either side of today's in the mortgage calculator or the compound interest calculator to see how much a change would matter.