An SPO (stake pool operator) runs a stake pool: infrastructure that produces blocks for the Cardano network on behalf of the ada holders who stake with it. In governance, SPOs form one of the three voting bodies, alongside DReps and the constitutional committee.
SPOs do not vote on every governance action. Their vote is required for a motion of no confidence, for updates to the constitutional committee, and for a hard fork initiation. They also co-vote on protocol parameter changes that touch security-relevant parameters. An SPO’s voting power is the stake delegated to their pool, which is independent of the governance delegation the same holders give to a DRep.