The darkest hour is a foretold day where the Transformers will face the threat of utter extinction. In that time, it is said that a Prime will rise from the ranks of the Autobots and open the Matrix of Leadership, using its power to "light our darkest hour" and eliminate the threat once and for all.
Generation One cartoon
As he lay dying in the aftermath of the battle of Autobot City, Optimus Prime, knowing his time had come, chose Ultra Magnus to be the new Autobot commander. Magnus at first resisted, feeling himself unworthy, but Prime sought to comfort Magnus by telling him of the prophecy of the Chosen One. As it turned out, the chosen one was in the room, and it was Hot Rod.
Much later, after taking the Matrix from Galvatron and becoming Rodimus Prime, he opened the Matrix of Leadership, asking it to "light our darkest hour." This caused Unicron to self-destruct and thus halted his attack on Cybertron, saving the Transformers.
A year later, Optimus Prime, who had been revived, used the Matrix to cure the galaxy of the Hate Plague, which had filled all beings with rage and caused them to attack others uncontrollably. As he did so, Prime also asked the Matrix to "light our darkest hour."
At the close of the Beast Wars, when the Maximals emerged victorious, Optimus Primal reflected on Dinobot II's heroism with a quote from the Covenant of Primus: "In the spark of an enemy, there will be salvation. And in the darkest hour, there will be a light."
In the climax of the first season of Transformers: Prime, Optimus Prime learns that Earth's core is actually Unicron. And to prevent Unicron from waking and destroying the Earth, Optimus travels to Unicron's spark and opens the Matrix of Leadership in order to return Unicron to stasis and save the Earth.
Ratchet said "Let us light our darkest hour!".