Blockchains are upgraded form of databases.

In a blockchain, different pieces of information/transaction are grouped together and stored as a chain of blocks. Each block is linked to 2 others blocks. One preceding it and one succeeding it. Similarly, each transaction is linked to other transactions forming a Merkle tree.

Contents of the blockchain are encrypted and stored in form of a hash. Such that only people with the private key can see the contents.

As all the blocks are encrypted and linked together, it becomes almost impossible to change the content of a block.

Also, it helps in the sharing of information between different organizations without compromising data security.

Blockchains can be programmed also. You can program to complete a particular transaction once set conditions are met.

This is just an overview of the blockchain. I hope it helps

To know more about this topic: How blockchain works