Reflexer is a platform where anyone can use their crypto collateral to issue reflex indexes. Reflex indexes are stable assets that are not pegged to anything, very similar to how the US Dollar is not pegged and it is still considered stable.
What is RAI?
RAI is a non pegged, ETH backed stable asset. It is useful as more "stable" collateral for other DeFi protocols (compared to ETH or BTC) or as a stable asset with an embedded interest rate.