The Secret that broke the bars of Einstein's Cage for humanity and yet also, so it's said, brought the Shiplords down on it. The First Secret is, for lack of better terminology, a system that allows for interstellar teleportation. The closest sci-fi equivalent to it would be the BSG jump drive. Although at best medium ranged in interstellar terms, it remains the only known method of breaking the light barrier. There are limits to it, however.
First and most important, First Secret drive technology cannot function within a certain radius of a star. The exact reasons for this are unknown, but given that the Shiplords themselves will not try to use FTL once inside the stellar exclusion zone, it appears to be a very legitimate limit.
The second limit is more variable, that of recharge time. All First Secret drives have some limit on their recharge, with the most advanced being those of the Shiplord War Fleets, whose recharge periods are measured in fractions of a second. Tribute Fleet ships, those most likely to be defeated by Tributaries and to have their drives scavenged, are only capable of a single jump every thirty to sixty minutes.