The "Flash Return Rate" (FRR) is a special kind of rate. It is not based on an agreed fixed rate; instead, it is based on the average of all fixed-rate positions, of all terms, weighted by their amount. The FRR is displayed on the market statistics page.
The FRR updates once per hour. The advantage of using the FRR is that return rates tend to follow the market action. Take the BTC/USD pair as an example; when the market price goes up, return rates for USD also tend to go up and similarly for BTC return rates when the market price goes down.
By using the Flash Return Rate, funding providers can then safely provide funding knowing that they will not miss opportunities if the market return rate goes up.