A conflict fought across time in the centuries before the 32nd century, after which the Temporal Accords banned time travel throughout the Federation.
The Temporal Wars were a conflict, or series of conflicts, waged by factions using time travel to alter history. They are generally connected to the Temporal Cold War seen in Enterprise, in which agents from different futures interfered in the 22nd century. The fighting grew so destructive that the powers involved agreed to stop.
The wars are referred to in the third season of Star Trek: Discovery, set in the 32nd century. After the conflict, the Temporal Accords prohibited time travel, and the technology needed for it was outlawed across the Federation. When Michael Burnham and Discovery arrive from the 23rd century using the Red Angel suit, they are breaking that ban, and the Federation official Kovich keeps a close watch on their knowledge of the past.
The Temporal Wars explain why, by the time of Discovery's later seasons, time travel has become forbidden and nearly unknown. They also connect the distant setting of Discovery to the wider history of the franchise, giving a conclusion to the time travel plots of earlier series.