A blockchain stores information about transactions between users or other changes to its database (conducted without intermediaries) in a way that they can be tracked by any user forever for maximum transparency Some participants, the miners, gather information about transactions and compile them in so-called blocks, using huge amounts of costly energy, and thereby secure that the order of all transactions ever made is immutably documented in the de-central database of which any user has a copy on his device. In exchange they receive precious bitcoin as incentive for protecting the network from corruption. Within this network, users can transfer values and store information without a chance of them being altered or contested ever again. The documentation of a transaction happens fast instead of taking several days, like e.g. a foreign bank transfer does. No government and no corporate in the world could ever influence or sabotage a blockchain network the size of the Bitcoin blockchain, as only the majority of participants can decide changes to the rules. Intentional falsification of data by a group of people is virtually impossible.
For a comprehensive summary of what blockchain is and how it works, click here.