Immutable is the opposite of mutable. Mutable means changeable, and so if something is immutable, it is unchangeable.
In the context of crypto, blockchain, and DLT systems, immutable generally refers to the concept that once a transaction has been confirmed on a ledger, the network protocol is designed to make that transaction unchangeable and irreversible. This is in stark contrast to centralized systems in traditional finance, where the ledger is not decentralized, and so the entity that controls it, such as a bank, can reverse a transaction if it so chooses.
DLT systems tend to achieve immutability to their transactions and ledgers through the security of their protocol coupled with decentralization.