A 51% attack is when more than half the computer power on a network is run by a single person (miner) or a single group of people (mining pool) and creates fraudulent blocks of transactions for themselves while invalidating the transactions of others in the network. A 51% attack is more likely to arise in Proof of Work (PoW) network like bitcoin.
TRON uses an improved Delegated Proof of Stakes system (TRON PoS – TPoS). With a TPoS, the attacker would need to obtain 51% of the cryptocurrency to carry out a 51% attack. The attacker have to accumulate enough coin to apply for Super Representative candidate position, then outvote all TRON community voters and replace themselves with more than half of the existing 27 Super Representatives. Although it would be difficult and expensive to accumulate 51% of a reputable cryptocurrency, an attacker with 51% stake in the coin would not have it in their best interest to attack a network in which they hold a majority share.