Chainlink is a blockchain-based middleware, acting as a bridge between cryptocurrency smart contracts and off-chain resources like data feeds, various web APIs, and traditional bank account payments.
This way, Chainlink allows Smart Contracts to communicate with external resources on their own. LINK is an ERC20 token based on the Ethereum Blockchain.
It is used to pay Chainlink Node operators for the retrieval of data from off-chain data feeds, formatting of data into blockchain readable formats, off-chain computation, and uptime guarantees they provide as operators. The Chainlink token is also used as a collateral for node operators, which prevents bad actors.