What is Blokchain?

Think of Blockchain as stored information, like a spreadsheet or ledger. (Let's call it a BLOCK). This BLOCK contains DATA, a new HASH and a HASH from the previous BLOCK. (BLOCKs connect each other, creating a chain).
The Data depends on the type of Blockchain. The most hyped Blockchain is Bitcoin.

Bitcoin's data records the details of the sender, the receiver and the number of coins. The next bit of information is called a HASH.
HASH is what identifies the BLOCK and what the BLOCK is. Think of it as a big number key that's one of a kind. The last bit is the HASH from the previous BLOCK.

This is what makes Blockchain a unique way to verify a transaction between two parties that don't know each other. Another critical security feature: Blockchain is distributed on a peed-to-peer network.
Which means the BLOCKs are on the network of computers as opposed to one central location. So when a new BLOCK is created, i is sent to everyone on the network to verify.

Once it has been verified, it is added to the Blockchain. This is what's called consensus. The agreement to what's a valid BLOCK and what's not. Some future uses for Blockchain include an e-notary system that could store medical records and taxes. It could even render complex 3D graphics.

Source: https://computer.howstuffworks.com/what_is_blockchain.htm