Also called alternative history attack. This attack has a chance to work even if the merchant waits for some confirmations, but requires relatively high hashrate and risk of significant expense in wasted electricity to the attacking miner.
The reason for mentioning the 51% rate is that in order for this attack to succeed, you send your coins to a third party, withdraw your money, and then reverse the transaction history and for this scenario to succeed, you need at least 3 confirmations, meaning that you will mine 3 consecutive blocks faster than the rest of the network Something that will not happen unless it controls more than half of the network hashrate.
Explanation better ---->
https://en.bitcoin.it/wiki/Majority_attack