A distributed ledger is a database that is consensually shared and synchronized across multiple sites, institutions, or geographies, accessible by various people. It allows transactions to have public "witnesses". The participant at each node of the network can access the recordings shared across that network and own an identical copy of it. Any changes or additions made to the ledger are reflected and copied to all participants in a matter of seconds or minutes.
A distributed ledger stands in contrast to a centralized ledger, which is the type of ledger that most companies use. A centralized ledger is more prone to cyber-attacks and fraud, as it has a single point of failure.