A 51% attack is an event in which one individual or a collaborating group gains control over more than half of the entire hashing power of a blockchain network. In other words, the security of the network is directly related to the fact that no one is exceptionally powerful through the possession of that power.
In case one surpasses the fifty percent threshold, he/she/they will be in a position to influence which transactions are validated. For instance, they can slow down the other miners, prohibit the addition of new blocks, or even reverse payments they made–this is called double-spending.
Massive blockchains like Bitcoin or Ethereum cannot be attacked easily, since the amount of power required would cost billions of dollars to acquire and maintain. On the other hand, the smaller and newer blockchains could be the ones most at risk, as there have been several occurrences of such attacks on them in the past (like on the Ethereum Classic Blockchain).
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A 51% attack is a trigger for a full discussion of the advantages of decentralization in cryptocurrency. The more dispersed the computational power is, the more difficult it becomes for anyone to alter the past or to seize control from the community that supports the system.