A blockchain is a decentralized public ledger which is immutable meaning once the transaction details are written to a block it can't be changed by anybody. The copy of the ledger is with everyone on the network.
Because it's decentralized, so that removes the need for a 3rd party and the dependence on third parties. This also brings down transaction times and cost of transaction.
It's public so it's transparent.
It's immutable so that makes it a trust-less technology which when implemented in real life scenario's makes fraud impossible.