A confirmation means that your transaction has been included in a block. More confirmations means that the transaction is being buried deeper in the block chain. Because it is possible to create a fake block chain more than one confirmation is needed. If the block was fake, it will be rejected before a few confirmations.
[/attempt at non-technical explanation]
So if I understand this correctly, it is considered a lot harder to fake a longer chain then a small chain, so the longest chain wins[1].
Would it be possible to insert a fake block, and then trick the other miners that this is the real block, and they should generate a new block based on this fake one?
Notes
[1] hence the concerns with the 51% attack.
https://bitcointalk.org/index.php?topic=52388.0https://bitcointalk.org/index.php?topic=53647.0etc