If more than half the computer power or mining hash rate on a network is run by a single person or a single group of people, then a 51% attack is in operation.
A 51% attack refers to a malicious actor (or group acting in concert), controlling over 50% of the total mining power of the blockchain network and disrupting the integrity of the blockchain.
Due to the way blockchain functions, consensus is required for transactions to be confirmed or added. A bad actor who controls the majority of the hashing or mining power can theoretically "form the majority" in this consensus mechanism and disrupt the integrity of the blockchain by modifying the order of transactions, preventing transactions from being confirmed, or double spend.
The risk of a 51% attack is higher for blockchains with less hashing power, as it is easier for a malicious actor to procure the required majority computing power. The more miners and resources spent mining a blockchain, the safer the blockchain is. The Bitcoin network is recognized as the safest blockchain in existence as it has the largest amount of hashing power mining it.
An example of a 51% attack happened in January 2019 on the altcoin Ethereum Classic blockchain.