EOS is a blockchain platform for the development of decentralized applications (dapps), similar to Ethereum in function. In fact, supporters have dubbed it the Ethereum killer. It makes dapp development easy by providing an operating-system-like set of services and functions that dapps can make use of.
The idea behind EOS is to bring together the best features and promises of the various smart contract technologies out there (e.g. security of Bitcoin, computing support of Ethereum). Simply put, the EOS community is working to provide one simple to use, massively scalable dapp platform for the everyday user.