A blockchain is a decentralized database. It is distributed on a network of thousands of servers.
A new entry has to be verified by the majority of them. After this, all servers of the network update their version of the database. When one block is complete, the next one is generated. Due to the chronological storage of data in the individual blocks of the chain, these cannot be changed subsequently without damaging the entire system. The data on the blockchain is encrypted and manipulation is largely restricted. You can see that a transaction took place, but no private information is published. The blockchain is accessible to everyone worldwide. It became well-known as the technology behind the digital currency Bitcoin.