What is immutable in Web3?

Could you please clarify what exactly is meant by "immutable" in the context of Web3? I'm particularly interested in understanding how this concept applies to blockchain technology and the overall decentralized ecosystem. I've heard about the importance of immutability in maintaining the integrity and security of digital transactions, but I'm still a bit hazy on the specifics. Could you break it down for me in simple terms? What makes something immutable in Web3, and why is it such a crucial aspect of this new internet era? I'd really appreciate your insights into this matter.