Think of it as the backbone to the whole thing. It's the ledger, and it's permanent. Even when you edit something - a post or a reply - all versions of the post are recorded on the blockchain. It's a perfect, unhackable version of the history of things that occur. I say unhackable because it's not just kept as a record on a single machine. There are numerous machines, all keeping a copy of the blockchain (ledger) on their machine, and all transactions must match the info on the ledgers of the other machines on the blockchain, which are all scattered around the world. To hack the blockchain would mean all machines on the blockchain must be hacked, and this would be basically impossible.