Chainlink is the first and most widespread Oracle service. Many decentralised applications need to read data outside the blockchain. And because they cannot trust a single data provider, they need an intermediary to read the data from different sources and thus provide the most reliable data possible to the decentralised applications. This is the role of Oracle services.
Chainlink dominated the first wave of DeFi applications built on top of Ethereum, and has achieved significant collaborations with centralised applications inside and outside the cryptocurrency space.
A significant part of its operation is not decentralised as it is controlled by the company Chainlink. Newer Oracle services (Band protocol and API3) that offer decentralised services are pressing this weakness.