Chainlink uses what is called oracles to transfer information from off-blockchain sources onto on-blockchain smart contracts. It can effectively be seen as a decentralised data feed without the need for a trusted third party.
Chainlink decentralized oracle networks provide tamper-proof inputs, outputs, and computations to support advanced smart contracts on any blockchain. This allows for a reliable, tamperproof network, seamless connection to any API, and highly reliable and trust less automation on-chain.
Chainlink greatly expands the capabilities of smart contracts by enabling access to real-world data and off-chain computation while maintaining the security and reliability guarantees inherent to blockchain technology.