The Temporal Defense Grid is a satellite network which prevents unauthorised temporal incursions.
The grid was developed at some time in the 25th century, although the exact dates are unknown. It was also unknown to the temporal powers who created the grid - it went online simultaneously throughout the Federation, Klingon Empire, Typhon Pact, Regnancy of the Carnelian Throne and other Alpha Quadrant powers, and was quickly copied by other powers. The technology features many elements of different powers' technology, many as red herrings.
The grid prevents unauthorized temporal incursions, "bouncing" the attempted traveller back to their point of origin, although it allows temporal communication and agreed time travel. This is why the Temporal Cold War is a "cold" war - the factions fighting are protected from direct assault, and so must fight in their pasts.
The grid was developed in secret, and implemented in secret, in order to keep it out of the history books and so prevent temporal powers from travelling in time and undoing its creation. However, it was revealed that the project was created by Department of Temporal Investigations agent Gariff Lucsly, who found out about the grid from 31st century Temporal Agent Jena Noi, thus creating an ontological paradox. Lucsly organised a meeting with temporal investigation agents from many 24th century powers in order to propose the project, noting that they were "unimportant" - not presidents or starship captains, not people who would be remembered by history, thus allowing temporal security. (DTI novel: Watching the Clock)