Hi Chimaijem,
Blockhain is essentially a distributed digital ledger that can be used to record transactions in a transparent way. Without getting into too much details (since most of this information can be sourced online), it is usually deployed on a decentralised network and it requires updates to it to be verified and validated by consensus. This in-built checking mechanism is what makes it immutable (difficult to hack) and therefore secure.
Cryptocurrency leverages blockchain technology as do many of the modern international money transfer systems available.
