Blockchain is basically distributed ledger technology which means that transactions are recorded and distributed over all the blocks in the chain. Transactions cannot be reversed and are updated across the entire network

One way to think of it is like an SQL database on a web server. The web developer that designed the SQL database could change anything in the database to his wishes. Blockchain solves this issue by being distributed and making sure that every transaction on the blockchain is agreed upon by all the participants in the chain. Essentially, if one block is changed on the blockchain, it would be immediately become apparent that it was hacked.

It uses an immutable cryptographic signature called a hash to record transactions. This is basically a very secure signature that is not vulnerable to attacks. This is the reason for the term cryptocurrencies and also why most all transactions are anonymous over most blockchains