A Time portal was a gateway though which a person could time travel.
In the 31st century, one of the way a Temporal agent could travel back in time was through time portals.
When Daniels took Jonathan Archer to the future to protect him, he destabilized the timeline, destroying the future he knew. This also meant Earth never developed time travel technology, so there were no time portals to send Captain Archer back to 2152. (ENT: "Shockwave, Part I")