Uniswap is a decentralized exchange on the Ethereum Network. It allows anyone to create a pool with a pair of tokens which can be exchanged against each other. Once the pool is created liquidity can be added - which means that sufficient tokens of each pair need to be added - so that the community can start trading. Whoever added liquidity to the pool will earn a small transfer fee for every transaction in the pool. As the community adds more liquidity, the pool becomes 'deeper' and price volatility of the token pair becomes less.