What is a 51% attack in cryptocurrencies?

In a very large-scale Sybil attack, scammers take control of most of the computing power or hash rate of the network. In this case, a 51% attack can occur. Under such circumstances, attackers can change the order of transactions and prevent their approval. While the situation is under the control of scammers, they get the opportunity to cancel some transactions, which leads to double-spending.