Looks like a "preprocessor" on steroids. Imagine the C preprocessor, but instead of simple macros and constants you can define expressions that will be expanded

Looks like a "preprocessor" on steroids. Imagine the C preprocessor, but instead of simple macros and constants you can define expressions that will be expanded to much more complex things. This example they give is pretty good at conveying the idea: you can define "goto" to emit the intermediate LLVM opcodes to jump to a label, then use it immediately after. This is great for defining your own DSL.