Proof-of-History (PoH) is a blockchain synchronization algorithm that has an internal clock that shows the same time on all nodes. Network nodes work on a schedule, and thanks to synchronization, they can verify the time between transactions in the blockchain. So, using PoH allows creating a historical record that proves that an event occurred at a certain point in time.
For example, this algorithm is used in the Solana blockchain. While other blockchains require validators to communicate with each other to agree that time has passed, each Solana validator maintains its own clock by encoding the passage of time and using sequential hashing delay functions (VDFs).
Proof-of-History is a fundamental step forward in the structure of blockchain networks in terms of speed and throughput.