The Blockchain is a data storage principle according to which individual data blocks ("Block") are concatenated together over time ("Chain"). A basic principle is decentralized storage, i.e. the block chain is not stored on one single computer, but on several computers at the same time (this is basically redundant storage).
The principle of concatenation as well as the principle of decentralization provides protection against manipulation, since information in subsequent data sets builds on previous data sets - and this information is also distributed over several computers. Manipulation of one data record would immediately make the block chain inconsistent, and such an attempt to change it would not be accepted. This is why the block chain found an early application in the crypto currency Bitcoin, since "forging" money is impossible.