51% attacks refer to malicious actors (or groups) controlling more than 50% of the total blockchain mining power and disrupting the integrity the blockchain.
Because blockchain works, transactions must be approved or added to confirm them. The majority of hashing and mining power is controlled by bad actors. They can theoretically “form” the majority in the consensus mechanism to disrupt the integrity of blockchain transactions.
Blockchains with lower hashing power are more at risk for 51% attacks. This is because it is easier for malicious actors to obtain the majority of the computing power needed. Blockchains that have more miners or resources are safer. Because it has the highest amount of hashing power, the Bitcoin network is considered the most secure blockchain.
An example of a 51% attack occurred in January 2019 on altcoin Ethereum Classic.