LINK is a cryptocurrency in ChainLink which is a decentralized oracle service, and the first of its kind that allows businesses to create a smart contract easier by retrieving the data from the existing Oracle Database via API, or other off-chain sources then integrate into the blockchain (put it on-chain).
The problem with smart contracts is, per their design, smart contracts can only manage data on the blockchain. Their potential, the ability to provide tamper-proof decentralized applications for uses over the world, is still largely untapped. Many smart contract programs are built on Ethereum but they still lack a bridge to the real world industries. This is where ChainLink comes in.