Imagine you're building a house. You want it to be secure (safe from burglars), big enough for your family (scalable), and affordable (decentralized, meaning no one person is in charge). The blockchain trilemma is like this situation, but for blockchains. These are three important qualities for a blockchain: decentralization (no single point of control), security (resistant to attacks), and scalability (ability to handle a lot of transactions). The challenge is that it's difficult to achieve all three perfectly at the same time.
