The term “Blockchain Trilemma” refers to the widely-held idea that decentralised networks can only offer two out of the three advantages of decentralisation, security, and scalability at any given moment. There will therefore need to be a sacrifice made in one area.
For example, the blockchain trilemma faced by Bitcoin. We all know Bitcoin is the most secure and decentralized blockchain. However, in the term of scalability, Bitcoin has a poor reputation because of its less-than-ideal transaction speeds. It means, Bitcoin maximized their decentralization and security, while sacrificing their scalability.
Vitalik Buterin, Co-Founder of Ethereum was the first one to popularize the term blockchain trilemma when their team was facing this issue as they were developing Ethereum. Currently, many blockchain development teams are bringing innovation in the form of Layer-1, Layer-2, etc, to solve the blockchain trilemma.