Imagine a secure online record book where everyone can see what's written but no one can change it. That's kind of how blockchains work, but there's a potential weakness. A 51% attack is like taking over that record book by controlling more than half the people who verify new entries.

Here's a breakdown:

Blockchain: A digital ledger of transactions that's shared across a network of computers.

Mining: The process of verifying new transactions on a blockchain and adding them to the record. Miners use computers to solve complex puzzles, and whoever solves it first gets to add the next block of transactions.

Hash rate: The combined computing power of all the miners on a blockchain network.

In a 51% attack, someone (or a group) gains control of more than half the mining power on a network. This gives them immense power because they can:

Block legitimate transactions: They can prevent honest miners from adding new transactions to the record.

Reverse transactions: In theory, they could potentially rewrite past transactions, like taking back cryptocurrency they had spent. (This is difficult and risky, but it's a concern).

Because of this risk, it's important for blockchains to have a lot of miners spread across the network. The more miners there are, the harder it is for someone to launch a successful 51% attack.