A 51% attack refers to a bad actor who gains control of the majority of CPUs in a cryptocurrency mining pool or the overall hash rate. Hashing secures the blockchain. Any entity that obtains 51% of the overall hash rate AND gets 6 confirmations, has the power to rewrite the transaction ledger, roll back transactions, and spend them again (i.e. Double Spend). Without the 6 confirmations, then the entity can only censor or prevent. Such attacks are generally limited to smaller blockchains with fewer nodes (than the Bitcoin blockchain) because they’re more susceptible to a single-person seizing control based on a Proof of Work (PoW) consensus mechanism.
Glossary