Some languages require higher level internal representations ("IR"s) to be "lowered" to lower level representations before code generation - in other words like high level semantics or syntactic sugar and transform into simpler constructs to compile.
For example, a language could support special iterator semantics that will be "lowered" into a simple for loop. Or a for loop itself may be lowered into a do-while loop, depending on the design of the lowered IR.
To give you an example, rustc has an AST, and a high-level intermediate representation tree called the HIR. The former can encode anything that is syntactically valid (and some things that aren't), while the HIR only encodes things that are valid and transforms things like `if foo {} else {}` to `match foo { true => {} false => {}}`. That makes this internal representation more fundamental, there are fewer constructs to care about, and allows unifying some validations that would otherwise have to be written for all the alternative user visible constructs. It also makes later stages of the compiler not have to care about how code was written, just about this higher level representation.
Some languages require higher level internal representations ("IR"s) to be "lowered" to lower level representations before code generation - in other words like high level semantics or syntactic sugar and transform into simpler constructs to compile.
For example, a language could support special iterator semantics that will be "lowered" into a simple for loop. Or a for loop itself may be lowered into a do-while loop, depending on the design of the lowered IR.
To give you an example, rustc has an AST, and a high-level intermediate representation tree called the HIR. The former can encode anything that is syntactically valid (and some things that aren't), while the HIR only encodes things that are valid and transforms things like `if foo {} else {}` to `match foo { true => {} false => {}}`. That makes this internal representation more fundamental, there are fewer constructs to care about, and allows unifying some validations that would otherwise have to be written for all the alternative user visible constructs. It also makes later stages of the compiler not have to care about how code was written, just about this higher level representation.