That's mean a single entity is having 51% hashing power. Through this way, people can double spend a single coin. That means when you have 51% hashing power, you can reverse a transaction. That's it.
now every coin that are working on the proof of work algorithm can be attacked by 51%. Of course, except for Bitcoin. because doing this attack on bitcoin is useless.
Have you read the OP? Attacking in bitcoin is useless because it is the most active chain.