As an adjective, it means having a lot of money and expensive possessions; wealthy. As a noun (less common), it can mean a wealthy person or a stream that flows into a larger river.
From Latin *affluens* “flowing toward, abundant,” from *affluere* “to flow to,” built from *ad-* “to” and *fluere* “to flow.” The sense of wealth came from the idea of money ‘flowing in’ abundantly.
‘Affluent’ people are literally those with resources flowing toward them like a steady river. The link between flowing water and flowing money shows how we picture wealth as a constant current, not just a pile.
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