why is flexible rate of exchange called free rate of exchange?

The answer given by Pratik is correct. Flexible exchange rate system is also know as free rate of exchange as it is determined by the free market forces of demand and supply of foreign exchange. For further explanation on the same, follow the given link.

https://www.meritnation.com/ask-answer/question/how-is-the-exchange-rate-determined-under-a-flexible-exchan/open-economy-macroeconomics/6315164