Floating the currency is leaving it to the mechanisms of supply and demand, without interference from the central bank in determining its price. This floating is absolute, but what is common is managed floating, that is, in which central bank intervenes to direct its exchange rate.
Countries resort to a policy of floating their currencies in the event that they suffer from unstable financial and economic conditions. This was used in what is known as the currency war that the global economy witnessed after the 2008 international financial crisis.