Immutable means that something is unchanging over time or unable to be changed.
In the context of blockchain, distribution refers to a decentralized data storage model. Instead of one central database where all data is stored and subject to compromise, blocks of information are stored simultaneously in multiple locations and computationally locked together.
Hence, immutable distribution constitutes the heart and soul of blockchain. Once data is stored in a blockchain network, it can never be changed without alerting the rest of the network.
More data blocks can be added, but earlier data blocks are continuously confirmed as unchanged. Immutable distribution represents an improved paradigm of data storage.