For a real explanation, my last post was not precise enough. If you want an exhaustive explanation, better go to the Bitcoin wiki.

In short: Transaction malleability refers to a flaw in Bitcoin-based cryptocurrencies where an attacker could create the exact same transaction multiple times, but with different transaction hashes. This was used by attackers to fool merchants to accept a duplicate transaction in spite of the "real" one, but it has only consequences if the merchant does not wait until the transaction is confirmed (included in a block).

This was solved in Cryptonite already in 2014 ("unmalleable transactions" refer to TX which are not affected by transaction malleability, of course). Bitcoin only offers truly "unmalleable" transactions since Segwit was approved in late 2017. So XCN was no less than three years ahead