The easiest way to think about Blockchain is to imagine it as a hierarchical database that stores information in blocks of information that are interconnected by encryption and expanding over time. Each information block contains information about the time it was created and linked to the previous block, including a time code and transaction data. It's like a ledger.
But the special feature of Blockchain is that this database is distributed decentralized on the internet. That means that this database is spread around the world, owned by all computers in the peer network.
There is no central server hosting this database, no one has control over this database. Once the data is written to this ledger, no one can delete it, or change it. It can not be hacked. Blockchain helps participants reach consensus on information without going through a third party.