The temporal displacement policy was a series of guidelines instituted by Starfleet which obligated its officers to take no action which could alter the course of history if they were to travel through time.
After having accidentally traveled back in time from 2371 to 2024, Julian Bashir expressed his dissatisfaction with the temporal displacement policy when informed by Commander Sisko that they could do nothing to help the hundreds of people who would be killed in the Bell Riots, which would occur in only three days time. (DS9: "Past Tense, Part I")
In the non-canon novel Federation, the policy also specified that if a starship from the past were to encounter a starship from the future they should maintain communications silence unless the future starship hailed them and reduce the viewscreen's resolution to its lowest point. This particular note was contradicted in TNG: "Cause and Effect". It also stated that if the ship from the past was facing a situation in which it was not likely that it could be saved, then the starship from the future could take the crew of the past starship on board. This is consistent with the reaction of the alternate USS Enterprise-D crew in TNG: "Yesterday's Enterprise", as the temporally displaced USS Enterprise-C was initially invited to rejoin Starfleet in the 24th century rather than returning back through time to their near-certain demise. It was not until additional information came to light that the extraordinary decision was made to send them back anyway, despite the overwhelming likelihood of destruction.