A temporal vortex is a rift in the spacetime continuum. It allows a vessel to travel through time.
An example of such an occurance happened in 2373, when a Borg sphere created such a vortex to prevent the launch of Zefram Cochrane's Phoenix in the year 2063. The USS Enterprise-E followed the sphere back in time, and prevented it from stopping Earth's first contact with the Vulcans. Geordi La Forge recreated the vortex, allowing the Enterprise to return to the 24th century. (Star Trek: First Contact)