In Blockchain, the control of more than half of the hash power, that is, 51%, falling into the hands of a malicious group is referred to as a "51% attack". However, to properly understand this concept, it's first necessary to provide some explanation about blockchain-based systems.

The most important feature of Blockchain technology and Bitcoin is its decentralization. In blockchain-based systems, the creation and verification of data proceed in a completely decentralized manner. The operation of devices, known as “nodes”, that keep a copy of the transactions occurring on the blockchain network and are connected to the internet, ensures the system's decentralization. It's necessary for the majority of nodes to agree on issues such as mining, the version of software used, and the validity of transactions.

The power used by a computer or hardware to run and solve hash algorithms is referred to as "hash power" or "hash rate". In Proof of Work (PoW) based systems, a miner's performance is entirely dependent on their hash power. There are many mining nodes on the Blockchain, and miners compete with each other to find the next valid block hash to be rewarded with Bitcoin.
Thanks to PoW, the verification of a new transaction block is realized through the joint decision of nodes, thereby ensuring the accuracy of hashes. The decentralized ledger and distributed system prevent individuals or groups from using the network for their own benefit.
The decentralization of Blockchain was mentioned earlier. Ensuring that the hash rate is not controlled by a single person or group, and that mining power is distributed across different nodes worldwide, provides security without being dependent on a centralized authority. The concept of a 51% attack, or majority attack, is largely theoretical, predicated on the risk of not distributing the hash rate sufficiently. Let's examine the potential outcomes of a person or group having control over more than 50% of the hash power.

If any malicious user or group gains control over more than 50% of a blockchain's total hash rate, they could invalidate the network's consensus mechanism, leading to harmful actions such as double spending. In the event of an attack, the attackers would be able to change the order of transactions and even deny transactions, actions that require mining power. Moreover, those in control could prevent other users from mining at any time, leading to a mining monopoly.
Assume a malicious user manages to control more than 51% of the Bitcoin network's hash power. This person, by carrying out a 51% attack, could conduct offline OTC (Over the Counter) trading by sending Bitcoin to a crypto wallet in exchange for real money. Thus, the malicious user could transfer the buyer's money into their account upon transaction verification by the network nodes.
The attacker could then revert to a block in the blockchain before the BTC transfer was confirmed and mine on a version of the blockchain where the BTC transfer did not occur. Since the attacker controls more than half of the network, the system would recognize this otherwise impossible transaction as valid.

Although an attacker executing a 51% attack would have control over the system, they would not be able to prevent the sharing of transactions or reverse transactions. Additionally, the attacker would not have the authority to change block rewards, steal coins from users, or create new coins.
The deeper a swap transaction is buried, the more challenging it becomes to reverse it, as mining new blocks progressively distances them from the surface. This also explains why a certain number of confirmations are necessary when performing Bitcoin transactions.

The likelihood of a 51% attack on cryptocurrencies with a large network structure, like Bitcoin, is very low. This is because, as the network grows, it becomes increasingly difficult for a single attacker to acquire the hardware or resources needed to control more than half of the system. Therefore, 51% attacks on large networks, especially on the network of Bitcoin, which is considered the most secure cryptocurrency network, are highly unlikely.
Although most large blockchain networks have not yet experienced such an attack, smaller blockchain networks can be vulnerable to these types of attacks. For example, attackers carried out a 51% attack on Bitcoin Gold, a fork of Bitcoin, in May 2018, resulting in the theft of $18 million worth of BTG.