A blockchain is a database whose entries (blocks) are cryptographically chained, so altering history would mean redoing every later block across thousands of independent computers at once. That is economically infeasible. For hashtag.org, the practical point is that #Names recorded on the chain cannot be silently rewritten by a single party.
Web3 & crypto basics
Blockchain
A public ledger that records ownership and transactions in tamper-resistant blocks.