Blockchain is, simply put, a time-stamped series of immutable records of data that is managed by clusters of computers and is not owned by any single entity. Each of these blocks of data (i.e. block) are secured and bound to each other using cryptographic principles (i.e. chain).
The blockchain network has no central authority — it is the very definition of a democratised system. Since it is a shared and immutable ledger, the information in it is open for anyone and everyone to see. Hence, anything that is built on the blockchain is by its very nature transparent and everyone involved is accountable for their actions.
This helpful video explains it all, in just two minutes.