A blockchain is a distributed database that continuously grows with a record of all of the transactions that have occurred with a given cryptocurrency. This database is often referred to as a ledger because the data contains a large list of transactions that have taken place. In Monero, these transactions are packaged together into 'blocks' every 2 minutes (on average), and all miners and nodes on the network have copies of these blocks.
Unlike Bitcoin and other cryptocurrencies, transactions in the Monero blockchain do not reveal where funds came from or went to, providing anonymity and making the currency completely fungible. Additionally, the amounts of all transactions are hidden by RingCT, a feature of Monero. For auditing or other transparency purposes a user can share a view key to prove they control certain amounts of Moneroj.