The Chainlink network is a protocol that connects smart contracts to external data sources. It does this through the use of a decentralized network of nodes called oracles.
LINK is the native token of the Chainlink network and is usually used to provide real-world data (price feeds, weather, the final score of sporting events, etc.) to smart contracts in the Ethereum blockchain.
For more information on Chainlink, you may check their official website in this link.