"Yul" is an intermediate language that can be compiled to different backends such as the Ethereum Virtual Machine (EVM 1.0 and EVM 1.5) as well as Ethereum WebAssembly (ewasm).
Yul's goal is to be the common denominator for all three platforms. It's a good target for high-level optimization stages that benefit all target platforms. Yul can be used in stand-alone mode as well as "inline assembly" inside Solidity.
- Solidity Community