On Bitfinex, what is FRR? On Bitfinex under Lending it has a check box for FRR and FRR Delta
What do these option do? (Flash Return Rate)
Its the return rate. So instead of just setting a return rate at lets say 0.019%, you're return rate is the market rate. It updates once an hour. It makes it easier for you so you don't have to log in all the time to adjust you're rate.

Imagine. You set a rate to the current market rate of 0.049%. Then you get a few loans but the market rate drops to 0.019%. You no longer get loans so you have to adjust your rate. FRR adjusts your rate automatically.