Blockchain is a system that records all transactions in relation to the cryptocurrency. The system contains modules, each called “block”. Every block represents a decoded record of a specific cryptocurrency related transaction. It also contains a coded reference to the previous block, which consecutively contains a record regarding the previous transaction. As a result, all the information regarding transactions and manipulations done with certain units of a single cryptocurrency can be followed back to the original entry. By performing such backtrack - it’s validity can be verified. You need to consider that the blocks and entries cannot be altered once they are registered, making it impossible to impact / modify the whole chain, making it a safe and elegant solution to digital transactions.