What does RF mean?
RF stands for Reduction Factor. It means that the odds of your bet have been reduced.
An RF will be applied if, after you place your bet, a horse is withdrawn from the race.
The size of the RF will depend on the odds of the horse that is withdrawn. If the odds were low, the reduction factor will be high, and vice versa.
Both bookies and Betfair will apply an RF for withdrawn horses. The exact amounts may vary but they should be similar enough that your overall profit and loss will not be greatly affected.