A 51% attack refers to a situation where a single party gains control of more than half of a blockchain mining or validating power. With that majority, the attacker could potentially interfere with how new transactions are confirmed, for example by blocking some transactions or reversing their own recent ones.
There are limits to what such an attack can do. It cannot steal funds from arbitrary wallets or create coins out of nothing, because those actions still require valid keys and follow the network rules. The main risk is a specific trick called double-spending, where the attacker reverses a transaction they already made.
In practice, large networks are very hard to attack this way. Gaining majority control of a big blockchain would demand enormous resources, making it impractical and expensive. Smaller networks with less total power are more exposed, which is one reason network size contributes to security.
