More correctly... 1 Confirmation is the absolute bare minimum that you should accept as proof that a payment is complete. For higher value transfers (or for the more risk averse), you should consider waiting until a transaction has more confirmations before considering it "safe".

You would do well to read this: https://bitcoin.stackexchange.com/questions/1170/why-is-6-the-number-of-confirms-that-is-considered-secure

Essentially, the more confirmations, the more impractical it becomes to successfully execute a 51% attack.