One of the core features of blockchain.
Immutable is the inability to change something. Once it is completed, it is that way.
With blockchain, upon the validation of a block, the data contained therein cannot be changed, thus it is considered immutable.
This is the reason for Distributed Ledger Technology. Here each node agree on all information before moving forward. It is vital for cryptocurrency balances to alleviate the double-spend problem.
The key is decentralization which puts control of the network outside the hands of any one individual or entity.
A database built on a public blockchain cannot be destroyed. This means all records are safe from hackers or people will nefarious intent.