RRO are essentially banks giving money back to the Fed for a very short period in time (only a few hours - generally for overnight holding - these are called "overnight repos").

The thought is that RRO peak when the banks have zero use for the money (e.g the banks do not have any worthy borrowers to make additional loans to).

People have been looking at the RRO as a measure of excess liquidity in the market that even the banks don't want to hold onto cash. Others have argued that this is indicative of real negative interest rates taking place because the bank sees holding on excess liquidity as an expense to their business (although I am not sure that this has been proven)

here is a basic summary of the mechanics of a RRO: https://www.youtube.com/watch?v=H_wwzyAGPZw