Traffic Exchange Against AdSense ToS?

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Well I'm honestly really confused. I've been told by a few friends on here and also other people online that if I wanted to get good money from my Google AdSense ads I needed to join traffic exchange sites. Well someone just informed me it's supposedly against Googles Terms of Service to use traffic exchanges to increase their income. If this is true then how are people getting by with doing this and getting good money? Any advice will be appreciated, thanks :)