The sequence
Proof of History is a sequence of hashes, each computed from the one before it, recorded by a single leader. The point is not the cryptography — it is the ordering. The leader can stamp a transaction into the sequence at a particular slot, so other validators do not have to agree on the order after the fact.
Not consensus
It is tempting to read "Proof of History" as a consensus algorithm alongside Proof of Stake. It is not. Consensus is still stake-weighted voting through vote accounts; Proof of History is a clock that lets the leader speak in order. See the glossary entry for the one-line version.
Why it helps
With a shared clock, the leader can publish a stream of entries and the rest of the cluster can verify the ordering cheaply. This is what makes the high slot rate plausible — the network is not arguing about order, only about which fork to follow.