The blockchain is similar to a ledger that anyone can write in and which is unforgeable. It is impossible to alter, modify or delete the information in the register, let alone the register itself. This ledger, this database of transactions, is in fact shared between many users. It therefore becomes much more difficult to falsify.
Without going into too much technical detail, it is possible, thanks to this system, when generating an event or an interesting content, to transform this information into a unique sequence of characters called "hash".
For example, this 22-page file :
can be transformed into a hash (sequence of 64 characters) :
This hash is unique, any alteration of the file would lead to a different hash. Once the hash of an event or a file has been obtained, it is written (also called "anchored") as one transaction in one of the blocks of the blockchain. The file or event that gave rise to the hash is therefore not visible on the blockchain, but only its hash, which cannot be traced back to the original content (file).
Once the hash is anchored, it becomes possible to verify later that the file or event existed at a specific date T: it is only necessary to re-generate the hash at a later date T' to show that it is identical to the one that was originally produced T.
As the blocks of the blockchain are linked to each other in the chronological order of their validation, this chain architecture added to the decentralised nature of the blockchain is of paramount importance: if someone wanted to modify a transaction, he would have to alter not only the transaction, but also all the transactions that occurred after and before, up to the beginning of the chain.
It is impossible to assemble a centralized computing power capable of such an alteration.