51% Attack is when a malicious group controls 51% of the hash rate, and even if it is the wrong transaction, it will be approved as the right transaction if 51% of the people accept it as right. This is called a 51% attack.
The target of the attack is a virtual currency that uses an algorithm called Proof of Work (PoW), which is also used by Bitcoin.
PoW is the process of approving whether or not a transaction has been done correctly, and because PoW requires a huge amount of computation, it borrows the resources of PCs around the world to perform the approval process.
In PoW, the approval of a single transaction is not done by a single computer, but by several computers. This is called a decentralized network. If 51% or more of the computers in the network approve the transaction, the transaction is approved.
If 51% of the computers responsible for the approval process lie about their approval, the transaction can be tampered with. This is what is called a “51% attack”.
However, for now, there is no need to feel unsafe.
Bitcoin mining requires a large amount of computing capacity, which costs money in terms of machines and electricity. Even if it were possible to forge a transaction, the cost of attacking 51% of the transactions would be more.
Therefore, there is currently no advantage to attacking.