The whole point of using a blockchain is to let people—in particular, people who don’t trust one another—share valuable data in a secure, tamperproof way. That’s because blockchains store data using sophisticated math and innovative software rules that are extremely difficult for attackers to manipulate. But the security of even the best-designed blockchain systems can fail in places where the fancy math and software rules come into contact with humans, who are skilled cheaters, in the real world, where things can get messy.
To understand why, start with what makes blockchains “secure” in principle. Bitcoin is a good example.