There are many CSS preproccesors currently in the wild. They work creating a new language sweeted with syntactic sugar that either redefines CSS syntax or simply reuses it with some special meanings added. The document created is finally compiled so it can actually work on any browser.
LESS, as one of this preproccesors, uses the same syntax of CSS so the preprocessor can understand already existing CSS code; this existing code can actually work as a base for a new project. Even though the similar syntax, the preprocessor expects files with the extension .less to make its work.
Having said this, it should be noted that this is actually valid LESS code: