Proof of History (PoH) is a consensus mechanism most notably deployed by Solana, and is an evolution of the Proof of Stake model. As PoH employs Verifiable Delay Functions (VDFs) it is able to incorporate time itself into the blockchain, as a VDF can only be solved by performing a certain set of consecutive steps by a single CPU core.
As no parallel processing is permitted, it is simple to determine how long each step will take, and also calculate the time based on historical occurrences. After analysing these occurrences, a hash function is constructed that can be confirmed by anybody. Every block created by the network has this hash appended to it.
This is unlike traditional blockchains (where gaining consensus on the time a block was mined is just as important as getting consensus on the transactions in that block). While this PoH is drastically able to increase scalability with this method, it is still a relatively newer consensus mechanism that has yet to be as battle-tested.