Essentially, Blockchain is a type of database system that records and stores information in a secure manner that makes it generally impossible to hack, alter or in any other way defraud the system. It is perhaps most notably used in transactions involving cryptocurrencies like Bitcoin and Ethereum.
It stores data in 'blocks' which are then 'chained' together. As new data comes in, it is entered into a fresh block. Once the block is filled with data it is chained to the previous block, so that the data chained together is in chronological order, thus maintaining the integrity of the data sequence.