AggregatorV3Interface. In this case,
AggregatorV3Interfacedefines that all V3 Aggregators will have the function
latestRoundData. You can see all of the functions that a V3 Aggregator exposes in the
AggregatorV3Interfacefile on Github.
latestRoundDatato obtain the most recent round of price data. We're interested in the price, so the function returns that.