It means "the surrounding area or environment"; or "completely surrounding or encompassing". More recently, it has come to relate to creating a certain "reaction or mood, often a subconscious one, by being wherever people tend to be" (like ambient advertising); or to sounds that create a peaceful and relaxed atmosphere ( Garden Centre ).
Lastly, search engines do use the keywords in domain names as a ranking factor (which explains the existence of domains like "where-to-buy-garden-plants- North-London.com"),
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This means you can finally buy domains that you have had your eye on for a while but have previously not been able to as someone else has already registered them.