Uniswap is a decentralized protocol for automated liquidity provision on Ethereum. The protocol is one of the leading players in the Automated Market Maker (AMM) space where liquidity providers can deposit assets and earn swap fees. Users can tap into this liquidity and swap between ERC-20 tokens freely in a non-custodial manner. UNI is a token that will be used in the governance of the protocol, including the community treasury, protocol fee switch, and many more. 

More information here https://uniswap.org/