Low German is/was spoken in the northern lowlands near the sea, High German is/was spoken in the more mountainous regions in the south. This map shows quite well where the two languages are spoken, even though I think it's more a coincidence than causation that the language border matches the topological border.

Nowadays things have changed of cause, as Standard German is spoken in all of Germany, at least in official contexts. But in colloquial conversations and especially in the rural areas the difference persists.