What is 51% attack?
A blockchain is a distributed database where individuals or company’s running a node or wallet on the blockchain will have a copy of some or all of the database and the contents of that database retains consistency through consensus of every node storing that data.
A 51% attack is when 51% of all the nodes online provide consensus on records in the database of which they have altered. Because greater than 50% of nodes concur, the records from the 51% will be written as fact. This is why the larger a network of nodes the more difficult it is to alter the records. Due to Bitcoin having so many more nodes & miners it is seen as one of the most secure blockchains against the 51% attack. The smaller the hashrate and node count, the more susceptible a block chain is to a 51% attack. Though rare, they have been known to happen. One of the first was with the Verge crypto currency but there have been more to follow, here is an article outlining some of the victims.