A 51% attack occurs when a nefarious actor takes control of more than 51% of the validation authority of a blockchain network to approve invalid transactions or manipulate legitimate transactions for financial gain.
Blockchains require the majority of a network of validators to come to an agreement on its latest version. This process ensures the validity of information and is known as reaching consensus.
For established blockchain networks, 51% attacks are largely practically impossible and economically infeasible. An attacker would need to put together millions/billions worth of validator equipment and infrastructure to break the network, to the point that the value of their exploit would not justify the investment.
So blockchains are secured by the incentive structures to encourage at least 51% of the transaction validators to be honest. This incentive and validation method is necessary to establish the truth without the need to trust an intermediary.
Less established blockchain networks having a higher risk for 51% attacks exacerbates the compounding growth of established networks. More users result in better security, which results in more users, and the snowball keeps growing.