The so-called blockchain trilemma describes the difficulty in trying to maximize the three components of scalability, security, and decentralization. Even though blockchains are considered superior to traditional networks in many respects, no single blockchain is perfect.
The trilemma is that maximization on one of the components is hardly possible without compromising in one or even both of the other aspects. In scalability, one wants to optimize the ability of a blockchain to facilitate as high a volume of transactions as possible. Security, of course, one wants to maximize to protect the data stored in the blockchain from attacks of any kind. Through maximum decentralization, redundancy in the network is the goal to prevent a few participants from controlling large parts or even the entire network. It is important to mention that it is not necessarily impossible to achieve an optimum in these three components and therefore one cannot speak of a "law" or an unsolvable problem per se, but rather as an important issue in the construction of a blockchain.
