A hard fork initiation is a governance action that moves Cardano to a new major protocol version, introducing changes to the ledger rules that older node versions cannot follow. It is how protocol eras advance under on-chain governance instead of by developer decision alone.
It is the only action type on which all three governance bodies vote: SPOs, DReps, and the Constitutional Committee must each approve it before the upgrade proceeds. SPO approval matters practically as well as politically, since block producers must actually run the new node software for the fork to happen smoothly.