Saw some mentions about term "51% attack". So what does this exactly mean and how did ReapChain solve this problem?
A 51% attack means that a malicious attacker has over 50% of the network's total hashing power, creates new blocks at a faster rate than other honest nodes, and propagates them to the network.
That's true, basically, it is a hacking attack that forces you to adopt a blockchain that contains forged data.