The "Flash Return Rate" is a special kind of rate: it is based not on an agreed fixed rate, but on the average of all fixed-rate positions, of all terms, weighted by their amount, as displayed on the market statistics page.
The Flash Return Rate (FRR) updates once per hour. The advantage is that return rates tend to follow the action. For example, on the BTCUSD pair: when the price goes up, return rates for USD tend to go up, and similarly for BTC return rates when the price goes down.
Funding providers can then safely provide funding knowing that they will not miss opportunities if the market return rate goes up. Traders will find it even more interesting, because if the return rate they pay goes up, that usually mean their position is profitable.