forward rate agreements

(Financial Contracts) (a) What is a forward rate agreement (FRA)? Explain this financial contract in your own words. (b) John C Hull's writes in his book "Options, Futures and other Derivatives" that "A FRA can be valued by assuming that the forward LIBOR interest rate, IF, is certain to be realized". Explain this sentence in your own words. Hint: You can use a concrete example, assuming you have entered a FRA some time ago at no cost, paying a fixed rate r, and consider changed market conditions