A (bitcoin) blockchain is a database where you can rely on the records/entries to be legitimate since falsifying them is expensive and requires consensus. The older an a entry is, the more expensive it becomes to manipulate it.

In a traditional database, the records can be manipulated by the administrator of the database at will.

As you can imagine, for a currency system where no one should be in control, we can't use a traditional database where someone has admin access. We use a blockchain.