On-Demand Liquidity (ODL), also called Ripple Payments, Ripple's cross-border money-transfer system.
It moves money between countries by instantly converting one currency into XRP and then into the destination currency.
It uses market-makers to do a source-currency-to-XRP-to-destination-currency swap in one atomic step, avoiding the traditional need for banks to pre-fund accounts abroad (the Nostro/Vostro system); as it matures it is expanding beyond just XRP to include the RLUSD stablecoin and tokenized deposits.
It matters because it aims to replace the old, cash-heavy correspondent banking model with instant conversion, changing how cross-border payments work.