Blockchain is actually something as an authenticated data structure, which is a special linked list, where to create some of the hash pointers require a kind of a work to do or a kind of a monetary stake to risk. It means that we can consider the security of a given transaction which is mined in a block. This security will increase as the further blocks are minded on the top of the chain. The security is dependent on the exact consensus mechanism, but will increase somehow as follows:
Actually in a standard forking attempt, it is considered that a given transaction is secure if more than 6 blocks are mined on the top of the chain at Bitcoin and like 12-15 at the case of Ethereum. However if we an old transaction, than further added blocks increase the security further. It is certainly required at an UTXO style system, because old unspent transactions should also payed into account, however it is questionable if we really need it if we use an account based system like Ethereum.
