Immutability is the property of a record that cannot be changed or deleted once it has been written. On a blockchain, each block contains the hash of the one before it, so altering an old transaction would mean redoing all later blocks and outpacing the rest of the network. How this works in practice is explained on the Immutable Transaction page.
Immutability is strong but not absolute: it grows with each confirmation, and a 51% attack or a hard fork can, in rare cases, rewrite recent history.