Blockchain at a fundamental level is a chain of blocks. The blocks contain digital information and they are connected to each other using cryptographic keys. Due to its design, a blockchain network is resistant to modification. The most common usage is to record transactions between two parties in a verifiable and permanent way. Altering a record after it is registered on the network would mean that all subsequent blocks will also have to be changed. This would require consensus from majority of the peer-to-peer network members who validate and secure the blockchain network. Blockchain also happens to be the backbone of the digital currency 'Bitcoin'.
Also review the answer provided by Mr Mayank Gupta, Benchmark Six Sigma's in-house expert.