Since anything digital can be copied over and over again, the hard part about implementing a digital payment system is making sure that nobody spends the same money more than once. Traditionally, this is done by having a trusted central authority (like PayPal) that verifies all of the transactions. The core innovation that makes Bitcoin special is that it uses consensus in a massive peer-to-peer network to verify transactions. This results in a system where payments are non-reversible, accounts cannot be frozen, and transaction fees are much lower.