A 51% attack is an attempt to bring down a blockchain network made possible by taking control of the majority of its mining hashrate or computing power. For instance, a 51% attack on a Bitcoin blockchain can see miners take control of the majority of the hashrate, disrupting its operation. In such a scenario, the attacking miners can dictate which transactions to process and in what order and even possibly reverse them, resulting in double-spending.
When malicious attackers take over the majority of the mining hashrate in a successful 51% attack, they can cause a denial of service to users of the blockchain. The potential attack scenario can also cause attackers to change the block rewards, mint new tokens, or steal tokens from the blockchain.
