Oracles are data feeds that bring off-chain data on-chain so that the smart contracts that you build can query real-world information and build logic around it. For example, prediction market Dapps use oracles to settle payments based on events. A prediction market may ask you to bet your ETH on who will become the next president of the United States. They’ll use an oracle to confirm the outcome and payout to the winners.
Chainlink is the most popular oracle out there, you’ll usually use it to get price feeds, to get verifiable randomness, to call external APIs, etc. If you want to get started building with Chainlink, go to their documentation.
To learn more visit the Ethereum.org page on oracles.