51% attack refers to a group of miners trying to control more than 50% minning's power of the network.
By doing so they can block new transactions by taking place or being confirmed.Attackers use 51% attack to reverse the transaction which has taken place the phenomena which is known as double spend.Anyone who has a control over blockchain by controlling majority of hash powerd can reverse the transaction.For better understanding suppose someone use bitcoins to purchase a Car after the delievey of car logistics shows that bitcoin has been transfered.However upon 51% attack attackers are able to reverse the transaction so the buyer of car now owns bitcoins which he used to buy it.
Krypton and shift suffered 51% attack in 2016.
In May of 2018, Bitcoin Gold, at the time the 26th-largest cryptocurrency, suffered a 51% attack. The attackers were able to double-spend for several days, successfully stealing more than $18 million worth of Bitcoin Gold.
On the other hand, a form of a 51% attack is possible with less than 50% of the network's mining power, but with a lower probability of success.