Blockchain is just a decentralised database.
What is a database?
A database is a data structure used for storing information. This includes data that can be queried to gather insights for structured reporting used by entities to support business, financial and management decisions. Government also make use of databases to store large sets of data which scale to millions of records. A user with permissions associated with their account can change entries that are stored on this centralised database. By changing the ‘master copy’, whenever a user accesses a database using their computer, they will get the updated version of the database entry. Control of the database remains with administrators, allowing for access and permissions to be maintained be a central authority.
This is not at all the same as with a blockchain.
For a blockchain database, each participant maintains, calculates and updates new entries into the database. All nodes work together to ensure they are all coming to the same conclusions, providing in-built security for the network.
I have tried to explain Blockchain in many different ways. Do check out my other answers, let me know which one you think is the easiest to understand.