It looks like it's compiling TypeScript/Flow and newer JS features into more standard JS. Part transpiling.
Compiler here probably means transpiler. Like Babel. Supporting new ECMAScript stuff, transpiling it to ES3/ES5 or whatever, so you can use it in IE6, or on NodeJS 8.
> Compiler here probably means transpiler.
A transpiler is a type of compiler, so it's fine to just call it a "compiler".
'Transpiler' adds useful extra information and context though. It's like it's fine to just say 'animal' but it's a bit more useful with more information to say 'cow'.