A blockchain-based "smart contract" system with a Turing-complete contract programming language. Questions about Ethereum can also be asked on the dedicated Ethereum site: https://ethereum.stackexchange.com.
Ethereum is a "Bitcoin 2.0 / Web 3.0" distributed computing system.
Like Bitcoin, Ethereum uses a blockchain to store data and a virtual machine running a stack-based scripting language to act on transactions. Ethereum takes this further than Bitcoin by having a Turing-complete scripting language, with computing time allocated using a pay-per-instruction scheme.
The entire Ethereum network has the processing power of a 1999-era mobile phone, and given the costs of computing and storage on the network it should not be used for computationally or storage heavy tasks. Rather it should be used to publish and validate important state information.
For more information see - https://ethereum.org/