What does ls stand for on Chevy Impala?

On a Chevy Impala, "LS" doesn't stand for anything in the sense of an acronym or abbreviation related to a specific trim level or package. Instead, LS is the name of a *family* of General Motors engines. A Chevy Impala might have an engine *from* the LS engine family, but "LS" on its own doesn't describe a specific Impala trim. You would need to look at the full model designation to understand the trim level (e.g., Impala LS, Impala LT, etc.).