A 51% attack is a potential attack where a person or group gains a majority of a blockchains hashing power to control the network and its ledger.
How Does A 51% Attack Work?
Whenever a cryptocurrency exchange happens, the transactions are stored within a block and every couple of seconds or minutes this block is validated by a consensus of nodes or computers attached to the said network. A block is only added to the chain after it has been validated.
The blockchain contains a record of all exchanges that anybody can view at any time, known as a distributed ledger. This arrangement of record-keeping is decentralized, meaning no single individual or group has control over it.
But when a majority, or more than 51%, of hashing power is controlled by a single entity the network is disrupted.
The attackers would control the network meaning they’d be able to prevent some or all transactions from being confirmed, reverse completed transactions allowing double-spending coins, and prevent some miners from mining (through a mining monopoly).((https://academy.binance.com/en/articles/what-is-a-51-percent-attack))