Cryptographic immutability is the property of a record that cannot be altered without the change being obvious, because it is protected by cryptographic hashes. On a blockchain each block includes the hash of the block before it, so changing any old transaction would change every hash that follows. The general idea, and its limits, are explained in full in the Immutability entry.
The word "cryptographic" separates this from immutability enforced by rules or trust, such as a database whose administrator promises not to edit past entries. Hashing on its own makes tampering detectable; it is the network's consensus process, such as proof of work, that makes tampering very costly, because an attacker would have to redo the work for every later block faster than honest participants. The practical result for users is explained on the Immutable Transaction page.
Immutability is strong but not absolute. A 51% attack or a coordinated change to the software can rewrite recent history, and immutability also means mistakes and thefts cannot simply be reversed.